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Marine Biological Laboratory Library 



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Presented by 

ESTATE OF HERBERT W. RAND 
January 9, 1964 



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SMELL, TASTE, AND ALLIED SENSES 
IN THE VERTEBRATES 



MONOGRAPHS ON EXPERIMENTAL 

BIOLOGY 



PUBLISHED 

FORCED MOVEMENTS, TROPISMS, AND ANIMAL 

CONDUCT 

By JACQUES LOEB. Rockefeller Institute 

THE ELEMENTARY NERVOUS SYSTEM 

By G. H. PARKER, Harvard University 

THE PHYSICAL BASIS OF HEREDITY 

By T. H. MORGAN. Columbia University 

INBREEDING AND OUTBREEDING: THEIR GENETIC 
AND SOCIOLOGICAL SIGNIFICANCE 

By B. M. EAST and D. F. JONES. Bussey Institution. Harvard University 

THE NATURE OF ANIMAL LIGHT 

By E. N. HARVEY. Princeton University 

SMELL, TASTE AND ALLIED SENSES IN THE 
VERTEBRATES 

By G. H. PARKER. Harvard University 

BIOLOGY OF DEATH 

By R. PEARL. Johns Hopkins University 
IN PREPARATION 

PURE LINE INHERITANCE 

By H. S. JENNINGS. Johns Hopkins University 

LOCALIZATION OF MORPHOGENETIC SUBSTANCES 

IN THE EGG 

By E. G. CONKLIN, Princeton University 

TISSUE CULTURE 

By R. G. HARRISON. Yale University 

INJURY, RECOVERY AND DEATH IN RELATION TO 
CONDUCTIVITY AND PERMEABILITY 

By W. J. V. OSTERHOUT. Harvard University 

THE EQUILIBRIUM BETWEEN ACIDS AND BASES IN 
ORGANISM AND ENVIRONMENT 

By L. J. HENDERSON. Harvard University 

CHEMICAL BASIS OF GROWTH 

By T. B. ROBERTSON. University of Toronto 

COORDINATION IN LOCOMOTION 

By A. R. MOORE. Rutgers College 
OTHERS WILL FOLLOW 



? 

MONOGRAPHS ON EXPERIMENTAL BIOLOGY 

SMELL, TASTE, AND ALLIED 
SENSES IN THE VERTEBRATES 



BY 

G. H. PARKER, Sc.D. 

PROFESSOR OF ZOOLOGY, HARVARD UNIVERSITY 



37 ILLUSTRATIONS 





PHILADELPHIA AND LONDON 
J. B. LIPPINCOTT COMPANY 



COPYRIGHT. Ip22. BY J. B. LIPPINCOTT COMPANY 



Electrotypcd and Printed by J. B. Li ppincott Company 
The Washington Square Press, Philadelphia, U. S. A. 



EDITORS' ANNOUNCEMENT 

THE rapid increase of specialization makes it im- 
possible for one author to cover satisfactorily the whole 
field of modern Biology. This situation, which exists in 
all the sciences, has induced English authors to issue 
series of monographs in Biochemistry, Physiology, and 
Physics. A number of American biologists have decided 
to provide the same opportunity for the study of 
Experimental Biology. 

Biology, which not long ago was purely descriptive 
and speculative, has begun to adopt the methods of the 
exact sciences, recognizing that for permanent progress 
not only experiments are required but quantitative experi- 
ments. It will be the purpose of this series of monographs 
to emphasize and further as much as possible this develop- 
ment of Biology. 

Experimental Biology and General Physiology are one 
and the same science, in method as well as content, since 
both aim at explaining life from the physico-chemical 
constitution of living matter. The series of monographs 
on Experimental Biology will therefore include the field 
of traditional General Physiology. 

JACQUES LOEB, 
T. H. MORGAN, 

W. J. V. OSTERHOUT. 



AUTHOR'S PREFACE 

SENSE organs have always excited general interest, 
for they are the means of approach to the human mind. 
Without them our intellectual life would be a blank. The 
deaf and the blind show how serious is the loss of even a 
single set of these organs. 

Although the ear and the eye have commonly received 
most attention, the other sense organs, such as those of 
smell and of taste, are in reality equally worthy of con- 
sideration. These organs are of first significance in 
warning us of untoward conditions that may exist about 
us particularly in relation to our food. But they not only 
serve us in this protective way, they are also of the utmost 
importance in initiating that chain of events which cul- 
minates in successful nutrition. Through their action the 
secretion of the digestive juices and other like operations, 
so essential to the proper treatment of the food, are 
started and furthered in the alimentary canal. Thus 
their activities, though less associated with our mental 
states than are those of the ear and of the eye, are never- 
theless so essential to our organic well-being that they 
are in reality quite as necessary to us as the so-called 
higher senses. 

Smell and taste, together with certain other senses not 
so well known, form a more or less natural group in which 
there is a certain amount of functional interrelation and 
genetic connection, and it is from this standpoint that 
these senses will be considered in the following pages. 
They will thus illustrate in a way principles common to 

7 



8 AUTHOR'S PREFACE 

other groups of sense organs, and these principles will be 
found to be of an essentially dynamic character as con- 
trasted with the older conceptions in which function has 
been brought into relation less intimately with structure. 

The author is greatly indebted to the editors of this 
series of monographs for many suggestions that have led 
to improvements in the text. He is also under obligations 
to his wife for a careful revision of the manuscript. He 
wishes to extend his thanks to numerous persons who 
have permitted him to copy and use figures contained in 
their publications. In all such instances the sources of 
such figures are acknowledged in the text. Where a 
figure is given without reference, it is an original. The 
drawings for all figures were made by Mr. E. N. Fisher. 

G. H. P. 
Harvard University, Cambridge, Mass. 

January, 1922. 



CONTENTS 



CHAPTER PAGE 

I. NATURE OF SENSE ORGANS 13 

II. ANATOMY OF THE OLFACTORY ORGAN 23 

III. PHYSIOLOGY OF OLFACTION 42 

IV. VOMERO-NASAL ORGAN OR ORGAN OF JACOBSON 92 

V. THE COMMON CHEMICAL SENSE 102 

VI. ANATOMY OF THE GUSTATORY ORGAN 110 

VII. PHYSIOLOGY OF GUSTATION 132 

VIII. INTERRELATION OF THE CHEMICAL SENSES. . 167 



INDEX . 187 




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ILLUSTRATIONS 

FIG. PAGE 

1. Diagram of the Lateral Wall of the Right Nasal Cavity of Man . . 24 

2. Diagram of a Transverse Section of the Right Nasal Cavity of Man 25 

3. Respiratory Epithelium from the Nasal Cavity of a Young Pig. ... 27 

4. Olfactory Cleft of Man 28 

5. Olfactory Epithelium from a Pig Embryo 29 

6. Olfactory Epithelium from a Young Mouse 30 

7. Isolated Olfactory Cells and Sustentacular Cells from Man 31 

8. Isolated Olfactory Cell and Sustentacular Cell from a Frog 32 

9. Olfactory Cell of a Pike Showing Flagellum 33 

10. Olfactory Epithelium from a Chick Embryo 36 

11. Ventral View of the Head of a Shark (Scyllium) 38 

12. Diagram of the Right Nasal Cavity of Man Showing the Direction 

of the Inspired Air Currents 46 

13. Simple Rubber Olfactometer 50 

14. Double Olfactometer 51 

15. Ventral View of the Head of a Hammer-head Shark 66 

16. Curves of Olfactory Exhaustion 71 

17. Olfactory Prism 75 

18. Generalized Diagrams of the Molecular Structure of Classes 

of Aromatic Bodies (Olfactory Stimuli) 80 

19. Head of Human Embryo showing Vomero-nasal Pore 93 

20. Diagram of the Median Face of the Left Nasal Cavity of Man 94 

21. Transverse Section of the Snout of a Young Frog 95 

22. Transverse Section of the Head of a Snake Embryo 96 

23. Transverse Section of the Nasal Septum of a Young Cat 97 

24. Epithelium from the Vomero-nasal Organ of the Sheep 98 

11 



12 ILLUSTRATIONS 

25. Dorsal View of the Human Tongue 112 

26. Vertical Section of a Fungiforra Papilla 113 

27. Vertical Section of a Vallate Papilla 114 

28. Lateral View of a Catfish Showing Gustatory Branches of the 

Facial Nerve 116 

29. A Simple Taste-bud 117 

30. A Compound Taste-bud 118 

31. Taste-buds of the Rabbit 121 

32. Taste-buds of the Cat 122 

33. Taste-buds of the European Barbel 124 

34. Diagram of the Human Tongue Showing Innervation 125 

35. Diagram of the Possible Paths of the Gustatory Nerves in Man. . . . 126 

36. Diagrams of the Human Tongue Showing the Distribution of the 

Four Tastes 149 

37. Diagrams of the Receptor Systems of the Vertebrate Chemoreceptors 181 



SMELL, TASTE, AND ALLIED 
SENSES IN THE VERTEBRATES 

CHAPTER I. 

NATURE OF SENSE ORGANS. 

Contents. 1. Older Conception of Sense Organs. 2. 
Modified View due to Theory of Reflex Action. 3. The 
Genesis of Receptors. 4. Bibliography. 

1. OLDER Conception of Sense Organs. In the con- 
ventional text-book, sense organs are commonly looked 
upon as structures that supply the brain with those nerv- 
ous impressions from which the mental life of the indi- 
vidual is built. During normal activity these organs are 
incessantly in operation and flood the central apparatus 
with a stream of impulses by which are carried to us evi- 
dences of the multitudinous alterations of the environ- 
ment. Through the ear and the eye pass continuous 
streams of change by which we adjust ourselves not only 
to the immediate material world about us but to the 
world of ideas whose elements are spoken and writ- 
ten words. 

Sense organs from a structural standpoint are organs 
whose cells are so specialized that they are subject to stim- 
ulation by only a particular category of external changes. 
As Keith Lucas has expressed it, sense cells approximate 
a unifunctional state. The changes by which they are 
brought into action form rather homogeneous groups of 

13 



14 SMELL, TASTE, ALLIED SENSES 

environmental alterations. Thus the chemical changes of 
the surroundings affect the organs of smell and of taste, 
the pressure changes those of touch and hearing, and al- 
terations in the radiant energy those of sight. These 
natural groups of environmental changes have been des- 
ignated as homologous, or, better, adequate stimuli for 
the sense organ that they activate. Such organs are ordi- 
narily arranged under five heads each with an adequate 
stimulus and productive of a special sensation ; they are 
the organs of smell, taste, touch, hearing, and sight. 

Experience has also shown that when in a given per- 
son a sense organ exhibits complete congenital incapacity, 
such an individual lacks certain mental elements that can 
never in reality be made good to him by the activity of the 
remaining parts. A state of this kind implies a certain 
mental deficiency in the given individual. If a person has 
been blind from birth, no amount of description can 
supply to him the sensations of the wealth of color that the 
external world holds for the normal man. Where blind- 
ness is an acquired defect, the rememberance of the 
former color sensations as compared with the present 
deprivation, makes the state of deficiency still more pro- 
nounced. And in those rare cases where there is a 
unilateral defect in color vision with sight otherwise unim- 
paired, the subject can contrast most vividly the state of 
deficiency with that of normal completeness. Such con- 
ditions, which are known to occur not only in sight but in 
the other senses as well, have had a most profound influ- 
ence on the interpretations that naturalists have placed 
upon the states presented by the lower animals. 

It has been commonly assumed, and with no small 
show of reason, that where an animal is found to possess 



NATURE OF SENSE ORGANS 15 

an eye or an ear, for instance, it should be accredited with 
all the central nervous activities, sensations and the like, 
that accompany such an organ in man, qualified only by 
the degree of development to which the particular organ 
in the given animal has arrived. Conclusions based 
upon such a course of reasoning were commonly ad- 
mitted as valid by the workers of a few decades ago 
(Lubbock, 1882; Graber, 1884) and the text-books of that 
period in dealing with the sense organs of the lower 
animals discuss these parts ordinarily under the conven- 
tional five heads of the older human physiology ( Jourdan, 
1889). From this standpoint one of the lower animals is 
like a defective human being in that its full sensory ac- 
tivity falls short of that of the normal man. Or it may be 
compared to a person whose sensory development is un- 
symmetrical and whose relations with the surroundings 
have come to be predominant through a limited number 
of sensory channels rather than through all. 

It is likewise perfectly clear that a given animal, whose 
organization in general may be simpler than that of man, 
may nevertheless excede him in a particular sensory 
capacity and in this respect at least stand above him. It is 
commonly admitted that the dog far outruns man in the 
keeness of Ms sense of smell and it has long been known 
that cats hear tones of a pitch much too high for the human 
ear. These and other like examples show that though the 
senses of the lower animals are in general less efficient 
than those of man, the reverse is occasionally true. 

Moreover among some of the lower forms, sense or- 
gans have been discovered that are not represented in 
man. Thus fishes possess, in addition to the five classes of 
human sense organs, the so-called lateral-line organs. 



16 SMELL, TASTE, ALLIED SENSES 

Here then must be a wholly novel set of sensory relations. 
As to the sensations arising from these organs man can 
form no direct conception, for they are entirely outside the 
range of his experience. Hence Leydig, the discoverer of 
the sensory nature of these parts, wrote of them as organs 
of a sixth sense. Thus to the older workers the senses of 
the lower animals were like those of a human being that 
had suffered either curtailment or expansion even to the 
extent of excluding or including whole categories of 
stimuli. But quite aside from the question of the number 
and variety of these parts, is the opinion held by most of 
the early workers that the sense organs of the lower ani- 
mals are primarily concerned with providing the brain 
or corresponding structure of the given creature with that 
body of sensation which was supposed to represent all the 
significant changes in the effective environment. 

2. Modified View due to Theory of Reflex Action. 
The belief that sense organs were chiefly concerned with 
providing the brain with the elements of which the mental 
life is composed suffered an important limitation from the 
work of the physiologist. This limitation arose from the 
development of the idea of reflex action. Originating 
about the time of Descartes in the seventeenth century, the 
conception of the reflex action grew in time into a most 
important principle for the interpretation of nervous 
operations. It was at first applied to that form of 
nervous activity whose outcome is fairly constant and in 
a way mechanical in that it is unassociated with conscious- 
ness, but it was gradually extended to include those per- 
formances in which consciousness is involved and at 
present it commonly refers to any chain of nervous 
activity in which a sensory stimulation produces an im- 



NATURE OF SENSE ORGANS 17 

pulse that, after passage through the central nervous 
organs, results in action. 

From the beginning many reflexes were believed to be 
unassociated with consciousness and though this view was 
subsequently combated and the idea of the reflex extended 
to nervous operations that included an obvious sensa- 
tional element, it nevertheless remained true that a host 
of reflex operations could be pointed out that were with- 
out representation in consciousness. Thus the impulses 
that flow from the vestibular portion of the human ear 
and that are of the utmost importance in maintaining 
equilibrium provoke no obvious sensations and the vast 
flux of afferent nerve action that moves from the mus- 
cle to the spinal cord and that is so essential to the 
coordination of bodily movements, runs its course without 
exciting sensation. These and many like instances have 
made it clear that the reflex, even in the most special ap- 
plication of the term may as often be unassociated with 
sensation as associated with it. 

As the first step in every reflex is the excitation of a 
sense organ and as many reflexes are unassociated with 
consciousness, it must be admitted that sense organs, not- 
withstanding the name, are not always necessarily con- 
cerned with sensations. Many certainly have nothing 
whatever to do with such central nervous states. Thus 
it is doubtful if the normal activity of the sensory endings 
in our muscles and tendons is ever productive of sensation. 
In consequence of this condition a reasonable objection 
was raised to the term sense organ and it was proposed by 
Bethe (1897) to use in place of it the word receptor. 
Although the theoretic force of this objection has not 
always carried conviction, the term receptor has come into 

2 



18 SMELL,jTASTE, ALLIED SENSES 

common use and the emphasis that it places on the organs 
to which it is applied as receivers of environmental change 
rather than as originators of impulses to sensation is 
certainly a step in the right direction. 

Human receptors belong to one or other of two classes. 
Either they are concerned purely and simply with the 
excitation of reflex acts and take no part in the pro- 
duction of sensations, in which case they may be called 
activators, or they are at the same time effective in 
arousing sensations, the elements of the intellectual life 
and hence may be appropriately termed sense organs. 
All receptors belong to either one or the other of these 
classes though in some instances a certain degree of 
temporary vacillation occurs. Hence it may be that these 
classes exemplify in a way two receptive functions, one 
of which predominates in one class and the other in the 
other. How these functions are related can best be 
gathered from the genetic history of receptors. 

3. The Genesis of Receptors. Eeceptors such as the eye 
and the ear, the organs of smell and taste, and the more 
diffuse sensory equipment of the skin, are found in all 
the more complex animals. They abound in the verte- 
brates, the mollusks,the arthropods, and to a less extent in 
the worms. They may be said to occur even in the coe- 
lenterates, as, for instance, among the jelly fishes, though 
in the majority of these animals the receptors present a 
diffuse condition more like that seen in the vertebrate 
skin than in the vertebrate eye or ear. This diffuse state 
seems to be characteristic of the receptors in the simpler 
sessile invertebrates. The more complex animals such as 
are capable of active locomotion exhibit almost invari- 
ably specialized types of organs. 



NATURE OF SENSE ORGANS 19 

So far as the neuromuscular system of the inverte- 
brates is concerned, forms as low in the scale as the annelid 
worms appear to possess all the elements of the corre- 
sponding system in the vertebrates. Such worms may 
have specialized receptors, eyes and the like, often of a 
highly complex structure. They possess a well-differ- 
entiated central nervous system as represented in their 
so-called brain and ventral ganglionic chain. Finally, 
they have an abundant variety of specialized effectors 
in their various muscles, glands, and luminous organs. 
Their receptors, central nervous organs, and muscles are 
so related that reflexes can be demonstrated on them as 
readily as on vertebrate preparations. In other words, 
they possess in completeness, though in simple form, a 
working neuromuscular mechanism essentially like that 
of the higher animals. 

When, however, an examination of such forms as the 
ccelenterates is made, it is found that the coral animals, 
the sea-anemones, the hydroids, and the like, possess 
scarcely any trace of a central nervous apparatus. In 
these animals fairly well specialized sensory surfaces 
occur, whose nervous prolongations connect either imme- 
diately with the subjacent musculature or give rise to a 
nerve-net which in turn connects with the contractile ele- 
ments. Thus the receptor is applied to the muscle very 
directly and without the intervention of a central organ. 
Such an arrangement allows of simple reflexes, for, when 
the receptive surface is stimulated, the animal responds 
at once by an appropriate muscular movement. Thus if 
meat juice is discharged on the tentacles of a sea-anemone, 
these organs carry out vermiculate movements and the 
gullet opens; or if the pedal edge of the column is touched, 



20 SMELL, TASTE, ALLIED SENSES 

the whole animal contracts. The fact that meat juice 
will not excite the pedal edge of the column and that a 
touch applied to the tentacles is seldom followed by more 
than a slight local activity shows that the external surface 
of the sea-anemone, though generally receptive, is locally 
specialized. As a matter of fact this surface in degree of 
differentiation stands between a diffuse receptive surface, 
such as the vertebrate skin, and a specialized organ like 
the eye or the ear. 

In the literal sense of the word the outer surface of a 
sea-anemone is not sensory though abundantly receptive. 
There is no reason to suppose that the receptive areas of 
these animals are concerned with initiating impulses to 
sensation. They connect very directly with muscles and 
serve quite obviously as trigger-like organs by which the 
muscle is set in action. A careful examination of the 
activities of sea-anemones has failed to reveal any evi- 
dence, such as can be produced from the more complex 
animals, to show that these simple creatures possess 
central nervous functions. Such functions apparently 
have no part in their organized performances. Hence 
their receptors have nothing whatever to do with initia- 
ting impulses to sensation, but are limited in their action 
to the excitation of the muscles after the type of the most 
mechanical reflex. The presence in ccelentrates of eye 
spots, olfactory pits, statocysts and other such special 
receptors is, therefore, no indication that these animals are 
endowed with corresponding sensations, as many of the 
older workers believed, but this condition merely shows 
that their possessors are especially open to a particular 
stimulus. An eye spot does not mean that the animal pos- 
sesses sight, but that it is readily excited to action by light. 



NATURE OF SENSE ORGANS 21 

Thus of the two functions that have been attributed to 
receptors, the capacity to excite action and the ability to 
initiate impulses for sensation, the former is much the 
more widely distributed of the two and is without question 
the more primitive. 

Since sponges are known to possess muscles but are 
devoid of nervous tissue, it is probable that they represent 
a type of organization which in point of time preceded 
that in which the nervous elements arose. So far as can 
be judged these elements originated in connection with the 
previously differentiated muscle and as a special means 
of exciting it to contraction. This earliest nervous mate- 
rial must have been, therefore, essentially receptive in 
character and must have served as the source of the more 
obvious receptors of specialized types. Thus receptors 
must be regarded as the original form of nervous struc- 
ture, concerned in the beginning with the simple excita- 
tion of muscle (activators) and subsequently involved, 
after the development of the central organs, with that 
supply of impulses which yields the elements of the intel- 
lectual life (sense organs). 

The extent to which a natural group of receptors may 
undergo differentiation and yet maintain a striking degree 
of mutual interdependence can nowhere be better illus- 
trated than with the chemical receptors, the organs of 
smell and of taste. It is from this standpoint that the 
structure and function of these receptors will be con- 
sidered in the following chapters. 

4. BIBLIOGRAPHY. 

BEER, T., A. BETHE, und J. VON UEXKULL. 1899. Vorschlage zu einer 
objektivierenden Nomenklatur in der Physiologic des Nervensy steins. 
Biol. Centralbl, Bd. 19, pp. 517-521. 



22 SMELL, TASTE, ALLIED SENSES 

BETHE, A. 1897. Das Nervensystem von Carcinus maenas. Arch. mik. 

Anat., Bd. 50, pp. 460-546. 
GBABEB. V. 1884. Grundlinien zur Erforschung des Helligkeits- und 

Farbensinnes der Tiere. Prag & Leipzig, 322 pp. 

JOURDAJST, E. 1889. Les sens chez les animaux inferieurs. Paris, 314 pp. 
LUBBOCK, J. 1882. Ants, Bees, and Wasps. New York, 448 pp. 
PABKEB, G. H. 1910. The Reactions of Sponges, with a Consideration of the 

Origin of the Nervous System. Jour. Exp. Zool., vol., 8, pp. 1-41. 
PABKEB, G. R. 1917. The Sources of Nervous Activity. Science, vol. 45, 

pp. 619-626. 
PABKEB, G. H. 1919. The Elementary Nervous System. Philadelphia, 

229 pp. 



CHAPTER II. 

ANATOMY OF THE OLFACTORY ORGAN. 

Contents. 1. Nasal Cavities in Man. 2. Nasal Mem- 
branes. 3. Olfactory Epithelium. 4. Intermediate Zone. 
5. Polymorphic Cells. 6. Sense Buds. 7. Free-nerve 
Endings. 8. Development of Olfactory Nerve. 9. Com- 
parative Anatomy of Olfactory Organs. 10. Bibliography. 

1. NASAL Cavities in Man. In man the olfactory 
organs are paired and are situated one in each nasal 
cavity. Each of these cavities possesses an external 
opening, the anterior nans, and an internal one, the 
posterior naris or choana, which communicates with the 
pharynx. (Fig. 1). The two nasal cavities are separated 
by the nasal septum, a partly bony, partly cartilaginous 
wall, which forms a smooth median partition between 
them. The lateral walls of these cavities are thrown into 
a series of more or less horizontal folds, the nasal conchas. 
These are commonly three in number for each cavity 
though in some instances only two are present and in 
others a fourth, fifth or even a sixth can be discerned. 
Of the three conchas usually present the most ventral one, 
the inferior concha, is the largest and extends through 
much of the length of the cavity in a direction approxi- 
mately parallel to its floor. Immediately above the inferior 
concha is the somewhat smaller middle concha which 
is followed by the still smaller superior concha. When 
only two conchas are present, they are the inferior and the 

23 



24 SMELL, TASTE, ALLIED SENSES 

middle, the superior being absent. When a fourth concha 
is to be seen, it is found above and behind the superior. 
It has been designated the first supreme concha and it 




Fio. 1. Diagram of the lateral wall of the right nasal cavity of man. I, Inferior concha; 
2, middle concha; 3, superior concha; 4, first supreme concha; 5, second supreme concha; the 
apertures numbered C to 10 arc covered from sight by the conchro, but their positions are 
indicated by vertical lining; 6, aperture of the nnsolacrimal duct opening into the inferior 
meatus; 7, opening of the maxillary sinus (middle meatus); 8, opening of the frontal sinus 
(middle meatus); 'J, and 10, openings of the ethmoid cells, 9. into the middle meatus, 10, 
into the superior meatus; 11, opening of the Kustachian tube; 12, vestibule; 13, atrium; 14, 
choana; 15, frontal sinus; 10, sphenoidal sinus whose opening is indicated by an arrow; 17, 
olfactory region whose limits are marked by the dotted line. The vertical dotted line shows 
the plane of section from which Fig. 2 was drawn. 

may be followed by a second or even, a third supreme 
concha. According to Schaeffer(1920), the first supreme 
concha is to be observed in about 60 per cent of all adult 
human beings. 

The three conchae ordinarily present project from 



ANATOMY OF THE OLFACTORY ORGAN 



the lateral wall of each nasal chamber into its cavity and 
partly divide that cavity into three approximately hori- 
zontal passages: the inferior meatus under the inferior 
concha, the middle meatus under the middle concha and 
the superior meatus under the superior concha. (Fig. 2). 
The external naris leads at once 
to the first chamber of the nose, 
the vestibule, which connects 
almost directly with the inferior 
meatus, less directly with the su- 
perior meatus and through the 
so-called atrium with the middle 
meatus. Between the median sep- 
tum of the nose and the laterally 
situated conchas is a considerable 
space known as the common 
meatus. Dorsally this space is 
continuous with a narrow slit 
lying between the superior concha 
and the septum and called the 
olfactory cleft. All' these pas- 
sages and spaces communicate 
more or less directly and freely 
through the posterior naris or 
choana with the pharynx. 

In the bones about the nose in man are large paired air- 
spaces or sinuses that communicate with the exterior 
through the nasal cavity. These spaces, which have been 
very fully described by Schaeffer (1916), are of consid- 
erable size and are lined with a mucous epithelium con- 
tinuous with that of the nose. They are somewhat variable 
in number and connections and yet they fall more or less 




Fio. 2. Diagram of a trans- 
verse section of the right nasal 
cavity in man made at the plane 
indicated by the vertical dotted 
line in Fig. 1. 1, inferior concha; 
2, middle concha ; 3, superior con- 
cha; 4, nasal septum; 5, inferior 
meatus; 6, middle meatus; 7, 
superior meatus; 8, common 
meatus; 9, olfactory cleft (left 
side); 10, ethmoid cells; 11, 
maxillary sinus. 



26 SMELL, TASTE, ALLIED SENSES 

naturally into four sets, the maxillary, frontal, and sphe- 
noidal sinuses and the ethmoidal cells. Each maxillary 
sinus is a large space in the maxillary bone above the 
teeth. It opens by a considerable slit into the anterior part 
of the middle meatus. (Figs. 1 and 2). The frontal sinus,in 
the frontal bone also opens into the middle meatus at a 
point above and anterior to the opening of the maxillary 
sinus. Each sphenoidal sinus opens into the posterior end 
of the appropriate olfactory cleft in a region known as the 
spheno-ethmoidal recess. The remaining accessory nasal 
spaces, the ethmoid cells, are more or less variable ; some 
of them open into the middle meatus by several apertures 
well above the slit for the maxillary sinus. Others open, 
more commonly by a single aperture, into the superior 
meatus. In addition to these various openings, the naso- 
lacrimal duct, by which the lacrimal secretions from the 
eye are carried to the nasal cavity, opens between a 
pair of lips on the lateral wall of the inferior meatus near 
its anterior extremity, 

2. Nasal Membranes. The nasal vestibule is lined 
with a delicate continuation of the outer skin. The walls 
of the deeper part of the nasal cavity are covered with a 
mucous membrane which is divisible into two regions, the 
restricted olfactory region in the dorsal part of the cavity 
and the much more extended respiratory region embrac- 
ing the remainder of the cavity. 

The mucous membrane of the respiratory region is 
reddish in color and consists of a pseudo-stratified epi- 
thelium containing ciliated cells and basal cells backed up 
by a well developed tunica propria. (Fig. 3.) The cilia 
of this region lash towards the choana. The secretion 
covering the surface of the epithelium comes from numer- 



ANATOMY OF THE OLFACTORY ORGAN 




ous branched alveolo-tubular glands which contain both 
mucous and serous cells. 

The conchae of the respiratory region have long been 
known to be extremely vascular and to be possessed of a 
structure like that of erectile tissue. This is especially 
true of their edges. They can be 
excited through reflex channels to 
considerable enlargement and the 
swelling thus produced may be suffi- 
cient to close completely the respir- 
atory passages. It is believed that 
this high vascularity of the respira- 
tory region is concerned with the 
moistening and warming of the 
current of respiratory air. The 
secretions of this portion of the nose 
are also believed to be inimical to 
pathogenic germs and thus to afford 
a protection to the deeper parts against the invasion 
of disease. 

The olfactory region in man is yellowish in color as 
it is in the calf and in the sheep. In the dog and the 
rabbit it is of a more brownish hue. According to the 
older anatomists it was supposed to extend in man over the 
dorsal half or even more of the nasal cavity. Von Brunn 
( 1892 ) , however, claimed by a reconstruction from sections 
that the olfactory epithelium was much more restricted 
than had been originally supposed. According to this 
author only a small portion of the superior concha and a 
correspondingly small part of the nasal septum represent 
the unilateral area of distribution of the olfactory nerve. 
This area in one subject measured 257 sq. mm. and in an- 



I.....6 



Fio. 3. Respiratory epi- 
thelium from the nasal cavity 
of a young pig; b, basal cell; 
c, ciliated cell. After Alcock, 
1910, Fig. 2. 



28 SMELL, TASTE, ALLIED SENSES 

other 238 sq. mm. The more recent results of Bead 
(1908), however, show that in man the olfactory fibers 
spread from the dorsal portion of the olfactory cleft ven- 
trally over the superior concha almost to its free edge and 
correspondingly over the septum to about one third its ex- 
tent. (Fig. 4). The antero-posterior spread of the nerve, 

according to this author, is about twice 
that of its ventral distribution on 
either the concha or the septum; 
hence the whole area innervated by 
each olfactory nerve, if spread out 
flat, would be approximately square 
in outline and not far from 25 mm. 
FIO. 4. olfactory deft to a side, somewhat over twice the 

of man opened by turning MI -AT -r> 

the nasai septum (s) up- extent ascribed to it by von Bruiin. 




ward; the blackened area o /Mi? TI ' J.-L. T rrn ii> 

shows the distribution of 6. Olfactory iLpithelium. The olfac- 

the olfactory nerve. After " 

Fig ad 3i 1908 ' Plate "' tory epithelium has been an object of 

interest to histologists for a long 
time. As early as 1855 Eckhard stated that in the frog 
it was composed of two classes of elements, long epithelial 
cells and nucleated fibers. Which of these served as the 
endings of the olfactory nerve he was unable to say. In 
the same year Ecker discovered on the deep face of the 
olfactory epithelium a third class of cells subsequently 
called by Krause (1876) basal cells. (Fig. 5.) These 
three classes of elements were identified in a number of 
vertebrates and described by Schultze (1856, 1862) who 
expressed the belief that the nucleated fibers were sense 
cells and represented the true endings of the olfactory 
nerve though he was unable to demonstrate a connection 
between these cells and the nerve. 

It is probable that the connection of the so-called 



NATURE OF SENSE ORGANS 



olfactory cell with the olfactory nerve-fiber was first seen 
by Babuchin in 1872 who showed that in a gold-chloride 
preparation, fibers could be traced from the nerve to the 
cells that were suspected by Schultze to be sense cells. 
In 1886 this connection was demonstrated with much 
greater certainty in methylen-blue preparations by 
Ehrlich whose results were con- 
firmed the following year 
by Arnstein. 

Because of the transitoriness 
of methylen-blue preparations, 
the results of Ehrlich and of 
Arnstein were looked on with 
some suspicion till they were 
reproduced in Golgi preparations 
by a number of workers, such as 
Grassi and Castronovo (1889) 
on the dog, Ramon y Cajal (1890) 
on mammal embryos, Van 
Gehuchten (1890) on rabbits, von 
Brunn (1892) on man, Retzius 
(1892a, 1892b, 1894) on fishes, 
amphibians, reptiles, and mam- 
mals, and by many other later 
workers on various vertebrates. 
( Fig. 6 ) . The results of all recent 
students in this field of histology 
support the statement that the 

olfactory epithelium of vertebrates is composed of at 
least three classes of cells : basal cells, ordinary epithelial 
or substentacular cells, and sense cells from which the 
olfactory fibers take their origin. Thus the opinion of 




Fio. 5. Olfactory epith- 
elium from a pig embryo 
6H inches long; b, basal 
cell; 0, olfactory cell; s, sus- 
tentacular cell. After 
Alcock, 1910, Fig. 10. 



30 



SMELL, TASTE, ALLIED SENSES 



Sclmltze on this subject has been unquestionably and 
abundantly confirmed. 

The sustentacular cells are the chief supporting ele- 
ments of the olfactory epithelium. Each of these cells 
has a distal cylindrical portion that contains the yellowish 
or light brownish pigment so characteristic of the olfac- 




Fio. 6. Olfactory epithelium of a young mouse showing the olfactory cells and, to the right, 
two suatentacular cells. Golgi preparation. After Retzius, 1892a, Plate 10, Fig. 2. 

tory region. The nuclei of these cells are oval and con- 
stitute the outermost zone of nuclei in the epithelium. 
Their proximal portions are more or less irregularly 
compressed and branched, hence the outlines of these 
parts are commonly jagged. 

The basal cells form a single row of block-like elements 
on the proximal face of the olfactory epithelium. Their 
short branching processes extend distally among the other 
cells of the epithelium. 

The olfactory cells are the most numerous of the three 
classes of cells in the epithelium. Their nuclei are roundish 
with well marked nucleoli and form the extensive nucle- 
ated band between the distal zone of sustentacular nuclei 



ANATOMY OF THE OLFACTORY ORGAN 



31 



and the less distinct proximal zone of basal nuclei. Each 
of the olfactory nuclei is lodged in an oval cell-body. 
Proximally this tapers rapidly into a fine olfactory nerve- 
fiber which eventually enters the olfactory bulb of the 
brain. Distally the body of the cell extends as a somewhat 
coarser rod-like structure to the outer surface of the olfac- 
tory epithelium where it ter- 
minates in a small enlargement. 
This enlargement has been 
called the olfactory vesicle by 
Van der Stricht (1909) who 
ascribed to it a centrosomal 
origin and believed it to play 
a significant part in olfactory 
reception. The olfactory vesi- 
cle carries a cluster of proto- 
plasmic filaments, the olfactory 
hairs. (Fig. 7). These hairs 
are apparently extremely deli- 
cate and are easily destroyed; 
hence they have escaped obser-j 
vation by many workers. They were probably seen in the 
frog as early as 1855 by Eckhard, but they were first gen- 
erally identified and thoroughly studied by Schultze (1856, 
1862) in a number of vertebrates. Apparently they are 
never very numerous; Schultze (1862) found that in the 
frog there were five to six hairs on each olfactory cell 
(Fig. 8), and von Brunn (1892) and Kallius (1905) re- 
corded six to eight in man. Eetzius (1894) noted two to 
five hairs on each cell in the snake Tropidonotus. Ballo- 
witz (1904) found ten to twelve or more in Petromyzon, 
and Alcock (1910) states that in the pig the number varies 




Fia. 7. Isolated olfactory cells 
and sustentacular cells from man. 
After von Brunn, 1892, Plate 30, 
Fig. 4. 



SMELL, TASTE, ALLIED SENSES 



from five to eight, Because of their great delicacy the ol- 
factory hairs are probably seldom observed to their full 
length. Schultze (1856) described those of the frog as 
long, but Jagodowski (1901) has shown 
that in the pike the hair may be twice as 
long as the olfactory cell itself, (Fig. 9) 
and may reach from the distal end of the 
cell through the whole thickness of the 
superimposed slime. So delicate are the 
distal portions of these hairs that 
Jagodowski has proposed for them the 
name of olfactory flagella or lashes. In 
the opinion of this author the so-called 
olfactory hairs are only the proximal ends 
of these lashes, the distal part having 
disappeared in the course of preparation. 
The lashes can be demonstrated by means 
of the Golgi method or by osmic acid. 
These lashes are without doubt the true 
receptive elements of the olfactory cells. 
The secretion in which they are suspended 
and whose thickness they probably 
penetrate is produced by the numerous 
olfactory or Bowman glands whose ducts 
open out abundantly through the olfac- 
tory epithelium. 

4. Intermediate Zone. In the majority 
of vertebrates there seems to be a fairly sharp boundary 
between the respiratory epithelium and the olfactory 
epithelium. In some mammals, however, these two regions 
are separated by a considerable intervening area, known 
as the intermediate zone. This was first described by 



Fio. 8. Isolated 
olfactory cell and 
suatentacular cell 
from a frog. After 
Schultze, 1862, 
Plate 1, Fig. 4. 



ANATOMY OF THE OLFACTORY ORGAN 33 

Grassi and Castronovo (1889) in the dog, and subsequently 

was identified by Alcock (1910) in the pig. In this mammal 

the epithelium of the intermediate zone is thicker than that 

of the respiratory region and thinner than that of the 

olfactory region. Besides basal cells it possesses two 

types of epithelial cells, ciliated cells like 

those of the respiratory epithelium and 1 

non-ciliated sustentacular cells like those 

of the olfactory region. It also contains 

many olfactory cells, but these cells are 

not as numerous in the intermediate zone 

as they are in the olfactory region 

where they are said to make up about 

seventy per cent of the cells present. It 

is plain from the accounts given that 

the intermediate zone is a region of 



transition between the two chief nasal FIQ.Q 

preparation of an 

regions, the olfactory and the respiratory. a lf pike ry (E e sox) ro in 
5. Polymorphic Cells. In most verte- Srb.3? ffl 



brates the olfactory cells exhibit great process ne arl" 

. ,, . . ,, -.- ., , shown, but also the 

umtormity 01 structure. In the fishes, ion g peripheral oi- 

factory flagellum. 



however, Dogiel (1887) has called attention 
to a polymorphism among these elements, 
and he has described in addition to the ordinary type of 
spindle-shaped olfactory cell, cylindrical olfactory 
cells and conical olfactory cells. These three types 
have been identified by Morrill (1898) and by Asai 
(1913) in a selachian (Mustelus) and by Jagodowski 
(1901) in the pike (Esox). To what extent this 
polymorphism occurs in other vertebrates and how 
important it is for a right understanding of the action 
of the olfactory organ has not yet been determined. 

3 



34 SMELL, TASTE, ALLIED SENSES 

6. Sense Buds. In 1884 Blaue described what he be- 
lieved to be sense buds in the olfactory epithelium of 
certain fishes and amphibians. This observation was not 
confirmed by later workers and it appears, as Betzius 
(1892b) has remarked, that the so-called sense buds are 
not true buds but folds or bands of olfactory epithelium 
seen in transverse section. The buds subsequently de- 
scribed by Disse (1896b) in the nose of the calf and shown 
by him to be supplied by free-nerve terminations are be- 
lieved by this author to be concerned with taste rather 
than with smell. These structures, however, are claimed 
by Kamon (1904) not to be true buds but bud-like 
appearances produced by the mouths of the Bowman 
glands. If this is so, no sense buds of any kind are 
known in the olfactory epithelium of vertebrates. 

7. Free-nerve Endings. In 1889 Grassi and Castronovo 
with some uncertainty described from the epithelium 
of the intermediate zone of the dog what they regarded as 
free-nerve endings. Whether these were end-organs of 
the olfactory nerve-fibers or not, they were unable to 
determine. In 1892 similar endings were observed by von 
Brunn at the border of the respiratory region in man. 
Von Brunn believed these endings to be terminals of the 
trigeminal nerve and, apparently by mistake, mentioned 
Ramon y Cajal as their discoverer. Free-nerve endings 
in the olfactory region were subsequently recorded by 
Retzius (1892b) in the mouse and frog, by von Lenhossek 
(1892) in the rabbit, by Morrill (1898) in Mustelus, by 
Jagodowski (1901) in Esox, by Kallius (1905) in the 
calf, and by Read (1908) in the kitten. Morrill 's obser- 
vation for Mustelus has recently been confirmed by Asai 
(1913). Hence there seems to be no doubt that in addition 



ANATOMY OF THE OLFACTORY ORGAN 35 

to the olfactory cells, free-nerve endings occur in the ol- 
factory epithelium of vertebrates. 

The source of the nerve-fibers from which the free- 
endings of the olfactory epithelium arise is not definitely 
settled. The fact that these endings may be very near the 
outer surface of the olfactory epithelium shows that they 
are not due to the incomplete impregnation of fibers from 
the olfactory cells as was suggested by Van Gehuchten 
(1890). Free-endings like those in the olfactory region 
also occur in the respiratory region and here the only pos- 
sible source for them is the trigeminal nerve; hence it is 
probable that this nerve is also the source of the free- 
nerve endings of the olfactory region. This opinion is sup- 
ported by the observations of Rubaschkin (1903) who has 
shown that in certain portions of the olfactory epithelium 
of the developing chick the two sets of fibers, those from 
the olfactory nerve and those from the trigeminal nerve, 
take somewhat different courses and that the trigeminal 
fibers are the fibers that give rise to the free-endings. 
(Fig. 10). Thus such evidence as there is favors the 
opinion first expressed by von Brunn and subsequently 
reiterated by a number of investigators, that the free- 
nerve endings of the olfactory region are from the tri- 
geminal fibers. The vertebrate olfactory epithelium, 
therefore, has two types of nerve terminations, olfactory 
cells as the exclusive receptors for the olfactory nerve 
and free-nerve endings as the probably exclusive endings 
for the trigeminal nerve. 

8. Development of Olfactory Nerve. Since the fibers 
from the olfactory cells pass as olfactory nerve-fibers 
to the olfactory bulb and terminate there without direct 
connections with any other cells, the olfactory cells in the 



36 



SMELL, TASTE, ALLIED SENSES 



nasal epithelium must be their cells of origin, as in fact 
was shown to be the case for the chick by Disse (1896a, 
1897). Here the olfactory nerve-fibers have been demon- 
strated to grow from certain olfactory epithelial cells into 
the olfactory bulb, the epithelial cells acting in all respects 
like neuroblasts Bedford (1904). The trigeminal fibers 




Fio. 10. Olfactory epithelium of an embryo chick (ninth day) showing olfactory cells, 
sustentacular cells, and free-nerve endings of fibers from ganglion cells of the trigeminal 
nerve. After Rubaschkin, 1903, Fig. 3. 

must on the other hand grow from trigeminal ganglion 
cells into the olfactory epithelium there to terminate as 
free-nerve endings, but of this there is at present no di- 
rect evidence. 

9. Comparative Anatomy of Olfactory Organs. The 
nasal organs in the lower vertebrates are very different 
from those in man. In Amphioxus a single sensory pit 
slightly to the left of the median dorsal line of the head 
and connected with the anterior end of the nerve-tube is 
assumed to be an olfactory organ. If this is so, it is prob- 
able that this pit corresponds to the single median olfac- 



ANATOMY OF THE OLFACTORY ORGAN 37 

tory sac in the cyclostomes notwithstanding the fact that 
this sac shows evidence in its deeper parts of being a 
double organ. In consequence of single nasal openings 
Amphioxus and the cyclostomes are commonly contrasted 
with other fishes, and in fact with all other vertebrates, 
and are called monorhine. Those in which the olfactory 
organs are obviously paired have been designated as 
amphirhine. 

In the sharks and rays the paired olfactory pits are 
situated usually on the ventral side of the snout. (Fig. 11) . 
The single opening of each pit is more or less divided by a 
fold of skin into an anterior inlet and a posterior outlet 
the latter sometimes leading into the mouth. As the fish 
swims through the water and particularly as it takes 
water into its mouth in breathing, a current of water is 
passed through eadi of its olfactory sacs. In this way the 
olfactory organs become associated with the respiratory 
current, a condition that is more pronounced in the lung- 
fishes than in the sharks and rays, for in the lung-fishes 
the anterior apertures are external and form true anterior 
nares, and the posterior openings lie within the mouth 
and correspond to the choanas of higher vertebrates. In 
the highly specialized bony fishes, the paired olfactory pits 
are almost always on the dorsal aspect of the head and 
quite distant from the mouth. Each pit has two entirely 
separate openings, an anterior inlet and a posterior outlet. 
By means of these two openings a current of water enters 
and leaves each pit. This current is produced either by 
ciliary action within the pit ( Amiurus) or by the action of 
the muscles associated with the jaws and gills (Fundulus). 
In bony fishes, then, the olfactory pits are purely recep- 
tive and are in no direct way connected with the respira- 



38 



SMELL, TASTE, ALLIED SENSES 



tory current as they are in the sharks and rays, and in 
the lung-fishes. 

In the air-inhabiting vertebrates each olfactory sac 
possesses, as in man, an external inlet, the anterior naris, 
and a posterior outlet, the choana, opening into the mouth 
or the pharynx. The olfactory sacs are relatively simple 
in amphibians, but become progressively more compli- 




Fia. 11. Ventral view of the head of a shark (Scyllium) showing the olfactory pita in rela- 
tion to the mouth. 

cated in reptiles and birds, and vastly more so in mam- 
mals. Here the surface of the sac is enormously extended 
through the development of lateral folds or conchae which 
may be further complicated by the production of second- 
ary folds. In mammals the more ventral of these conchaa, 
those attached to the maxillary bone, are apparently not 
concerned with olfaction, but lie in the purely respiratory 
region of the nasal chamber. The more dorsal conchae 
those from the ethmoid bone, serve as olfactory surfaces. 
It has been shown that in some mammals, as for instance 
in Orycteropus, there may be upwards of ten olfactory 



ANATOMY OF THE OLFACTORY ORGAN 39 

conchae. Forms that possess these larger numbers of 
conchas are known to be keen-scented and are termed mac- 
ro smatic. Those in which the number of olfactory conchae 
is small, four or fewer, such as the seals, some whales, 
monkeys, and man, are known to be less acute of smell 
and are called microsmatic. Others again, such as the 
toothed whales, porpoises and the like, in which the olfac- 
tory organ has almost completely degenerated, are sup- 
posed to be devoid of olfaction and are called anosmatic. 
In such forms the nasal cavities have lost their original 
sensory function and have come to be of importance only 
in connection with respiration, a purely secondary relation. 

10. BIBLIOGRAPHY. 

ALCOCK, N. 1910. The Histology of the Nasal Mucous Membranei of the 

Pig. Anat. Rec., vol. 4, pp. 123-138. 
ARNSTEIN, C. 1887. Die Methyl enblaufarbung als histologische Methode. 

Anat: Ans., Bd. 2, pp. 125-135. 
ASAI, T* 1913. Untersuchungen iiber die Structur der Biechorgane bei 

Mustelus laevis. Anat. Hefte, Arb., Bd. 49, pp. 441-521. 
BABUCHIN, A. 1872. Das Geruchsorgan. Strieker, Handb. Lehre den 

Gcweben, Bd. 2, pp. 964-976. 
BALLOWITZ, E. 1904. Die Riechzellen des Flussneunauges. Arch. mik. 

Anat., Bd. 65, pp. 78-95. 
BOWDEN, H. H. 1901. A Bibliography of the Literature on the Organ and 

Sense of Smell. Jour. Comp. N enrol., vol. 11, pp. i-xl. 
BEDFORD, E. A. 1904. The Early History of thei Olfactory Nerve in Swine. 

Jour. Comp. Neurol., vol. 14, pp. 390-410. 

BLAUE, J. 1884. Untersuchungen iiber) den Bau der Nasenchleimhaut bei 
Pischen und Amphibien. Arch. Anat. Physiol., ana\t. Abt., 1884, pp. 
231-309. 
VON BRUNN, A. 1892. Beitrage zur mikroskopischen Anatomie der men- 

schlichen Nasenhohle. Arch. mikr. Anat., Bd. 39, pp. 632-651. 
DISSE, J. 1896a. Ueber die erste Entwickelung des Biechnerven. Sitzb. 

Oesel. Naturwiss. Marburg, 1896, pp. 77-91. 

DISSE, J. 1896b. Ueber Epithelknospen in der Regio olfactoria der Sauger. 
Anat. Heft, Abt. 1, Bd. 6, pp. 21-58. 



40 SMELL, TASTE, ALLIED SENSES 

DISSE, J. 1897. Die erste Entwickelung des Riechnerven. Anat. Hefte, 

Abt. 1, Bd. 9, pp. 255-300. 
DISSE, J. 1901. Riechschleimhaut und Riechnorv bei den Wirbeltieren. 

Ergab. Anat. Entu-ick., Bel. 10, pp. 487-523. 
DISSE, J. 1902. Riechschledmhaut und Riechnerv bei den Wirbeltieren. 

Ergeb. Anat. Enticick., Bd. 11, pp. 407-436. 
DOGIEL, A. S. 1887. Ueber den Bau des Geruchsorganes bei Ganoiden, 

Knochen-fisclien imd Aniphibien. Arch. mikr. Anat. Bd. 29, pp. 74-139. 
ECKEB, A. 1855. Ueber das Epithelium der Riechsehleimhaut und die 

wahrscheinliche Endigung des Geruchnerven. Ber. Gesell. Beford 

Naturwiss., Freiburg, (Zeit. wiss. Zool., Bd. 8, pp. 303-306.) 
KiKiiARD, C. 18")-">. I'eber die Endigungsweise des Gcruchsnerven. Beitrdge 

Anat. Phy&lol., Bd. 1, pp. 77-84. 
EHRLICH, P. 1886. Ueber die Methylenblaureaction der lebenden Nerven- 

substanz. Deutsche med. Wochcnschr., Bd. 12, pp. 49-52. 
GRASSI, V. und A. CASTKONOVO. 1889. Beitrag zur Kenntniss des Geruch- 

sorgans des Hundes. Arch. mikr. Anat., Bd. 34, pp. 385-390. 
.FACOKOWSKI, K. P. 1901. 7ur Frage nach der Endigung der Geruchsnerven 

bei den Knochenfischen. Anat. Am., Bd. 19, pp. 257-267. 
KALLIUS, E. 1905. Geruehsorgan. Bardeleben, Handb. Ana-t, Menschr, 

Bd. 5, Abt. I, Teil 2, pp. 115-242. 
KAMON, K, 1904. Ueber die "Geruchsknospen". Arch. mik. Anat., Bd.64, 

pp. 653-664. 
KRAPSE, W. 1876. Allgemeine und microscopische Anatomie. Hannover, 

581 pp. 
VON LENIIOSSEK, M. 1892. Die Nerveoiurspriinge und Endigungen im Jacob- 

son'schen Organ) des Kaiiiiiclu'ns. Anat. Anz. Bd. 7, pp. 628-635. 
MORRILL, A. D. 1898. Innervation of the Olfactory Epithelium. Jour. 

Comp. Neural., vol. 8, pp. 180-182. 
PETER, K. 1901. Die Entwiekelung des Geruclksorgans und Jacobsonsehen 

Organs in der Rciihe derWirbeltiere. Hcrtwig, Handb. Entunck. Tl'ir- 

beltiere, Bd. 2, Teil. 2, pp. 1-82. 
PETER, K. 1911. Entwickelung des Geruchsorgans. Ergcb. Anat. Entwick., 

Bd. 20, pp. 43-95. 
RAMON Y CAJAL, S. 1890. Origen y terminacien de las fibras nerviosas 

ol factorial. Gaccta sanitaria, Barcelona. 

READ, E. A. 1908. A contribution to the knowledge of the Olfactory Appa- 
ratus in Dog, Cat, and Man. Amcr. Jour. Anat., vol. 8, pp. 17-47. 
RETZIUS, G. 1892a. Die Endigungsweise des Riechnerven. Biol. Untcrs., 

N. F., Bd. 3, pp. 25-28. 



ANATOMY OF THE OLFACTORY ORGAN 41 

RETZIUS, G. 1892b. Zur Kenntniss der Nervenendigungen in der Riech- 
schleimhaut. Biol. Unters., N. F., Bd. 4, pp.i 62-64. 

RETZIUS, G. 1894. Die Riechzellen der Ophidier. Biol. Unters., N. F., Bd. 
6, pp. 48-51. 

RUBASCHKIN, W. 1903. Ueber die Beziehungen des Nervus trigeniinu&; zur 
RiechscMeimhaut. Anat. Anz. r Bd. 22, pp. 407-415. 

SCHAEFFER, J. P. 1916. Tlie Genesis, Development, and Adult Anatomy 
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SCHAEFFER, J. P. 1920. The Nose, Paranasal Sinuses, Nasolacrimal Pas- 
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CHAPTER III. 

PHYSIOLOGY OF OLFACTION. 

Contents. 1. Nerves of Olfaction. 2. Passage of Air 
through the Nasal Cavity. 3. Minimum Stimulus. 
4. Physical Condition of Stimulus, Gas or Solution? 5. 
Olfaction in Fishes. 6. Fatigue and Exhaustion. 7. Quali- 
ties of Odors. 8. Chemical Relations of Odors. 9. Inade- 
quate and Adequate Stimuli. 10. Olfactory Reflexes. 
11. Bibliography. 

1. NERVES of Olfaction. The olfactory region of verte- 
brates has been shown to possess olfactory cells as ter- 
minations of the olfactory nerve and free-nerve endings 
representing in all probability the trigeminal nerve. It 
has long been the opinion of investigators that the olfac- 
tory sense is mediated by the endings of the olfactory 
nerve, but this opinion has not been without its opponents. 
Thus Magendie, in a series of publications beginning in 
1824, came to the conclusion that the trigeminal nerve was 
the nerve of olfaction and that the so-called olfactory 
nerve was one whose function was wholly unknown. His 
opinion was based in part upon experiments on the dog. 
After the olfactory nerves of this animal had been cut, it 
was found still to respond to acetic ether and to ammonia. 
Even when blindfolded a dog with severed olfactory 
nerves would seize cheese or meat but it would not eat 
meat sprinkled with tobacco. It was pointed out by 
Magendie 's critics that many of the stimulating sub- 
stances used by him, such as ammonia and the like, not 

42 



PHYSIOLOGY OF OLFACTION 43 

only possessed odor but were irritants for mucous sur- 
faces generally and thus without reference to olfaction 
could call forth vigorous responses. Magendie, however, 
claimed that his results were not dependent upon these 
substances, but could be demonstrated by the use of non- 
irritants, such as lavender oil. 

Magendie 's opinion that the trigeminal nerve was the 
nerve of olfaction was opposed almost from the beginning. 
Eschricht in 1825 pointed to numerous cases of persons 
who were anosmic in consequence of the absence of the 
olfactory nerve or of its degeneration. Bishop in 1833 
described a case of paralysis of the trigeminal nerve in 
which there was, however, full retention of olfaction. 
Picht (1829) and Duges (1838), both of whom were incap- 
able of olfaction in the ordinary sense of the word, were 
nevertheless easily stimulated through their nasal mem- 
branes by the vapor of acetic ether, or of ammonia. Val- 
entin (1839) found that a normal rabbit would sniff the 
body of a dead one, but that a rabbit whose olfactory 
nerves had been cut would not thus respond. Schiff 
(1859) experimented on five pups, in four of which the 
olfactories were severed, the fifth being retained in a nor- 
mal condition as a control. After recovery from the op- 
eration, the four pups in which the nerves had been cut 
were unable to find the mother's nipples, and did not dis- 
tinguish between a man and the mother though they 
turned their heads away and sneezed when ammonia or 
ether was administered. Acetic acid stimulated them only 
when its vapor was very concentrated. These and many 
other similar results completely overthrew Magendie 's 
contention and showed that, though the trigeminal 
endings were concerned with the reception of what may be 



44 SMELL, TASTE, ALLIED SENSES 

called irritants, true olfaction was accomplished only 
through the olfactory terminals, which have to do with 
delicate perfumes, aromas, and the like, many of which 
were associated with food. 

The recognition in nasal stimulation of the two classes 
of substances, irritants acting on trigeminal terminals, 
and true odors affecting the olfactory endings, is of funda- 
mental importance, and the failure to appreciate this 
distinction is responsible in part at least for much of the 
confusion that exists in what has been written on the 
olfactory stimulus. As early as 1851 Frohlich pointed out 
this distinction and called attention to the fact that irri- 
tants or stimuli for the fifth nerve ordinarily induce 
vigorous reflexes, respiratory and the like, whereas true 
odors are in nature much milder and seldom call forth 
strong responses. It is quite possible that some materials 
are stimuli for both classes of end-organs; thus tobacco 
smoke not only carries with it an aroma or true odor but 
also acts as an irritant, These two actions, however, may 
depend upon different chemical substances in the smoke. 
Other stimuli such as oil of mustard or possibly ammonia, 
that are chemically much more homogeneous than tobacco 
smoke, may affect, nevertheless, both sets of receptors and 
thus exhibit the characteristics of both irritants and 
true odors. A revision of the so-called olfactory stimuli 
from this standpoint is much to be desired. 

2. Passage of Air through the Nasal Cavity. In ordi- 
nary respiration in man the passage of air through the 
nasal cavity does not necessarily excite olfaction at once. 
Sooner or later, however the odor may be slightly sensed 
after which a few deep breaths or sniffing movements 
are usually made, whereupon full stimulation ensues. 



PHYSIOLOGY OF OLFACTION 45 

The course that the current of air takes through the 
nasal chamber during quiet respiration has been studied in 
several ways. Paulsen in 1882 published the results of ex- 
periments 011 the human cadaver. He opened the nasal 
cavity by sawing through the head of a cadaver close to 
the median plane. Pieces of red litmus-paper were then 
placed on different parts of the nasal surface and the two 
halves of the head were brought together again. By 
means of a bellows attached to the trachea of the cadaver, 
the current of air that in life passes through the nasal 
chambers was imitated. This artificial current was 
charged with ammonia and thus a means was given of in- 
dicating the spread of the current by the location of the 
pieces of litmus-paper that changed from red to blue. As 
a result of this test it was found that the inspired air took 
a curved course from the naris to the choana. (Fig. 12). 
The highest part of this curve was near the middle of the 
nasal cavity, but this never reached a point as high 
as the olfactory cleft. When the current was reversed 
by causing it to enter at the choana and emerge at 
the external naris, as in expiration, the direction of the 
current was found to be much the same as in inspiration 
except that a somewhat lower course was followed. Thus 
in both inspiration and expiration the current of air is 
limited to what is more generally regarded as the respira- 
tory region of the nasal cavity, the olfactory region being 
essentially undisturbed. 

Paulsen 's results were confirmed in all essential par- 
ticulars by a number of later investigators including 
Franke, Zwaardemaker, Danziger, and Rethi, who worked 
on dead animals and human cadavers by methods not un- 
like those used by Paulsen. Franke (1893) sawed open 



46 SMELL, TASTE, ALLIED SENSES 

the head of a human cadaver in the median plane, replaced 
the nasal septum with glass and by means of an artificially 
produced respiratoiy current showed that smoke in its 
passage through the nasal cavity remained in the so-called 
respiratory region. He observed, however, that both in- 
spiration and expiration were accompanied by strong 




Fio. 12. Diagram of the right nasal cavity of man laid open and showing by arrows the 
direction of the inspired current of air over the nasal septum (right half of figure) and over 
the lateral wall (left half of figure). After Paulsen, 1882. 

eddies in the moving air. Kayser (1890) aspirated very 
light magnesia powder into the respiratory current of a 
quietly breathing normal subject and then inspected the 
nasal surfaces by means of a rhinoscope. The magnesia 
particles accumulated on the moist surfaces of the respir- 
atory portions of the nose and not on those of the olfac- 
tory region, thus confirming Paulsen 's results but by a 
method that was by no means so artificial as that employed 
by other workers. It may, therefore, 'be regarded as 



PHYSIOLOGY OF OLFACTION 47 

fairly well established that the current of air that sweeps 
through the nasal cavity in quiet respiration is limited 
chiefly to the non-olfactory portion of that cavity. Ac- 
cording to Paulsen and to Zwaardemaker this current 
even in its eddying effect does not rise above the lower 
edge of the middle concha or at most, according to 
Franke, the lower edge of the superior concha. This 
limitation is probably more pronounced in expiration 
than in inspiration. 

Although the experimental evidence does not show that 
the respiratory current spreads to the olfactory surface 
of the nose, odorous particles must in some way reach this 
situation. Zwaardemaker (1895) was led to believe that 
the diffusion of these particles played an important part 
in this process, but diffusion is a relatively slow operation 
and it is very doubtful if it is a significant factor in carry- 
ing the odorous material to the olfactory receptor. It 
seems more probable that the shifting pressures that 
accompany respiration and the slight eddies that are 
formed in the general current are responsible for a grad- 
ual change of air in the olfactory cleft. The change thus 
produced is probably too slight to be detected easily by 
the means heretofore employed in tracing the current and 
yet it may be sufficient to initiate such olfaction as occurs 
in quiet respiration. Olfaction thus once begun would 
naturally excite sniffing and this process seems to be 
entirely sufficient to account for a rapid change of air in 
the olfactory cleft whereby olfaction would be brought to 
full height. Thus air currents are certainly the chief if 
not the sole factors concerned with transporting the 
odorous particles to the olfactory membranes. 

The accumulation of odorous materials on the olfac- 



48 SMELL, TASTE, ALLIED SENSES 

tory surfaces may be much intensified by the condensa- 
tion of moisture within the nasal cavity. Zwaardemaker 
(1917) has called attention to the fact that a fog formed 
from a vaporized salt solution is very much less stable 
when it includes odorous substances than when it does 
not. This condition is believed to depend upon the elec- 
tric charges earned by the particles concerned, and 
Durand (1918a, 1918b) recently claimed that olf action 
is more or less dependent upon an appropriate hygro- 
metric state in the olfactory atmosphere and that what- 
ever facilitates the condensation of watery vapor there 
facilitates olfaction. 

Among the older physiologists Bidder (1844) main- 
tained that olfaction was possible on inspiration and that 
expired air could not stimulate the organ of smell. Paul- 
sen's observations show that this opinion is improbable 
and the direct test of breathing odorous air in through 
the mouth and out through the nasal cavity has de- 
monstrated that it is quite erroneous. The olfactory 
sensations produced on expiration are noticea.bly less 
than on inspiration and this is probably due partly to the 
lower course maintained in the nasal cavity by the ex- 
pired air and partly to the previous elimination of much 
of the odorous material by attachment to the moist sur- 
faces of the mouth, pharynx, and other parts over which 
the air passes on its way to the nasal chamber. Never- 
theless, as Nagel (1904) has pointed out, the odors of our 
food during mastication are the results of stimulating 
material that reaches the olfactory surfaces through the 
choanaB rather than through the external nares. The 
importance of these odors in promoting the various kinds 



PHYSIOLOGY OF OLFACTION 49 

of digestive reflexes, muscular, secretory, and so forth, 
has long been recognized. 

3. Minimum Stimulus. The common belief that the 
olfactory stimulus consists of minute material particles 
suspended in the air current of the olfactory organ is 
supported by the observation that odors may be carried 
on the wind in a definite direction many miles. Odors 
do not emanate from a given center and disperse in all 
directions as sound and light do. Moreover many sub- 
stances, such as arsenic, that are odorless under ordinary 
circumstances, give out an odor after they have been 
heated sufficiently to volatilize. The fact, discovered in 
1917 by Woodrow and Karpman, that the adaptation time 
for olfaction the time needed for an olfactory sensation 
to wane completely is directly proportional to the vapor 
tension of the odorous material shows that olfactory 
stimulation is due to the activity of gaseous particles. 
These and other like observations have led to the conclu- 
sion, now generally accepted, that the olfactory organs 
are normally stimulated by material particles, and not 
by disturbances of a non-material character. 

Some odorous bodies such as musk are well known 
to give out these material particles for a very considerable 
time without appreciably changing weight. From the 
standpoint of the receptor this indicates that olfaction 
is called forth by an infinite simally small amount of sub- 
stance, and measurements directed toward testing this 
question justify the conclusion. These measurements 
have been made in a variety of ways. 

One method of procedure is that of evaporating a 
given weight of odorous material in a known volume of air 
and then testing the air by sniffing it. This method lends 



50 SMELL, TASTE, ALLIED SENSES 

itself readily to the determination of absolute measure- 
ments but it is not so easily applied to questions involv- 
ing the comparison of odors. For the measurement of 
olfactory acuity, but especially for the comparison of 
odors, Zwaardemaker invented an ingenious piece of ap- 
paratus called an olfactometer. (Fig. 13). This consists 
of two tubes that slide one within the other and so shaped 
that one end of the inner tube may be applied to the 
nostril. The odorous material is carried on the inner 
surface of the outer tube. When the inner tube, which 
is graduated, is slipped into the outer one so as to cover 



i i i i i i i i r 

a 8765*321 

Olfacties 




FIG. 13. Simple rubber olfactometer. After Zwaardemaker, 1895, Fig. 14. 

completely its inner face and air is drawn, into the nostril 
through the tube, the odorous surface being covered gives 
out no particles and no odor is perceived. If, now, the 
inner tube is withdrawn a certain distance so that a 
given surface of odorous material is exposed to the cur- 
rent of air, odorous particles escape into the current and 
these may be sufficient in amount to call forth olfaction. 
By adjusting the inner tube in relation to the outer one 
whereby more or less of the odorous surface is exposed, 
a point can be found where minimum stimulation occurs. 
The amount of odorous substance delivered under these 
circumstances to the air current has been designated by 
Zwaardemaker as an olfactie, the unit of olfactory stimu- 
lation. Having determined for a given substance the 
area necessary for the delivery of one olfactie, doubling 



PHYSIOLOGY OF OLFACTION 



51 



that surface by an appropriate movement of the inner 
tube will produce a stimulus of two olfacties and so 
forth. Thus a graded series of measured olfactory stim- 
uli can easily be obtained. Further, by using outer 
tubes carrying different odorous substances, various com- 
parisons can be instituted as measured in olfacties. 
Moreover, a double olfactometer (Fig. 14) may be easily 







14. Double olfaotometer. After Zwaardemaker, 1895, Fig. 15. 

devised in that two single olf actometers may be combined 
so that one current carrying an odorous material of a 
given concentration may be introduced into one nostril 
and another carrying a second odorous substance of 
known concentration can be introduced into the other 
nostril, or both currents may be united and the odorous 
mixture thus produced can be let into one nostril. Thus 
a variety of comparisons may be easily made. 

Van Dam (1917b) has modified Zwaardemaker 's ap- 
paratus by applying the odorous material in the form of 



52 SMELL, TASTE, ALLIED SENSES 

a rod instead of a coating to the inside of a tube. The 
rod is made of paraffin mixed in a definite proportion with 
the odorous substance and the extent to which the rod is 
exposed in the olfactometer tube is a measure of the 
concentration of the odorous particles in the air current. 
Rods of metal, platinum, gold, or zinc, have also been 
used ; these have been charged by immersing them in an 
atmosphere of odorous material for a given length of 
time and then tested. The odorous particles gather on 
their surfaces and are subsequently freed. The success of 
this method makes it clear tha.t in the original evapora- 
tion method more or less of the odorous material must 
become ineffective in that it adheres to the walls of the 
container in which the evaporation is carried out. 

As a means of avoiding these and other difficulties 
Allison and Katz (1919) have recently employed in the 
testing of stenches a type of odiometer that for accuracy 
of work bids fair to replace most of the other devices. 
It consists of a number of Venturi-type flow-meters so 
arranged that a measured volume of air can be passed 
at a uniform rate through or over the chemical, and this 
air is then mixed with another measured volume of pure 
air also flowing at a uniform rate. The concentration of 
the chemical is measured by determining its loss in 
weight after a measured volume of air has passed 
through or over it. From this loss of weight and the to- 
tal volume of air with which the chemical has been mixed, 
the concentration in milligrams per liter of air is deter- 
mined. The mixture of air and chemical passes finally 
through a tube with a glass funnel at the open end. The 
funnel is placed over the nose of the person who by a sin- 
gle inhalation tests the mixture. The odors are rated ar- 
bitrarily as detectable, faint, quite noticeable, strong or 



PHYSIOLOGY OF OLFACTION 53 

very strong. By this means extremely accurate quanti- 
tative results can be obtained. 

In testing olfactory acuity the majority of workers 
have used the method of evaporating a known weight of 
substance in a given volume of air. By this method Val- 
entin (1848) found that 1/2,000,000 of a milligram of oil 
of rose per cubic centimeter of air was odorous. Assum- 
ing that 100 cubic centimeters of this mixture were nec- 
essary for olf action, he concluded that the total weight of 
oil of rose used in this operation was the very small 
amount of 1/20,000 of a milligram. Valentin also found 
that water containing 1/2,000,000,000 of its weight of 
tincture of musk had a perceptible odor whereas water 
containing only 1/3,300,000,000 of this tincture could not 
be distinguished from ordinary water. One gram of the 
odorous mixture called forth the characteristic smell and 
contained only 1/2,000,000 of a milligram of tincture 
of musk. 

More significant measurements were made by Fischer 
and Penzoldt (1886) on chlorphenol and mercaptan. One 
milligram of chlorphenol was evaporated in a room of 
230 cubic meters capacity and was thoroughly mixed with 
the air. This dilution called forth an unquestionable ol- 
factory sensation. It contained 1/230,000,000 of a milli- 
gram of chlorphenol per cubic centimeter of air or, if it 
is assumed that 50 cubic centimeters of air are the mini- 
mum needful for olfaction, the total amount of chlor- 
phenol necessary was found to be 1/4,600,000 of a 
milligram. By a similar method it was shown that 
mercaptan, a liquid with a penetrating garlic odor, could 
be recognized at a concentration of 1/23,000,000,000 of a 
milligram per cubic centimeter, a concentration that 
would yield 1/460,000,000 of a milligram for every 50 



54 SMELL, TASTE, ALLIED SENSES 

cubic centimeters of air. Notwithstanding this infinites- 
imally small amount of mercaptan, the quantity, just 
designated was estimated by von Frey (1904) to contain 
some 200,000,000,000 molecules of mercaptan. 

Passy (1892a, 1892b) has made similar minimum de- 
terminations for a number of substances and has shown 
that artificial musk, probably the most powerful of all 
known odorous materials, is about a thousand times 
stronger than natural musk. In his other determinations 
he found that olfactory acuity ranged in thousandths of 
a milligram per liter of air from camphor at 5 to vanillin 
at from 0.005 to 0.0005. The last determination may be 
expressed as equivalent to 1/2,000,000,000 of a milligram 
of vanillin in a cubic centimeter of air, a high dilution 
but still not so extreme as that already recorded by 
Fischer and Penzoldt for mercaptan. 

The details of the more important of Passy 's deter- 
minations are given in the following table in which ol- 
factory acuity, as measured by the minimum amount of 
substance that was stimulating to the several persons 
tested, is expressed in thousandths of a milligram per 
liter of air. 

TaUe I. 

Minimum concentrations for olfaction in thousandths of a milligram of 
suhstance per liter of air (Passy, 1892b). 

Substances Thousandths of a milligram 

Camphor 5. 

Ether 1. 

Citral 0.5 to 0.1 

Heliotropin 0.1 to O.u."> 

Cumarin 0.05 to 0.01 

Vanillin 0.005 to 0.0005 

Passy (1892c) has also determined the minimum con- 



PHYSIOLOGY OF OLFACTION 55 

centration necessary for the olfaction of a number of 
alcohols. These determinations have been recorded in 
millionths of a gram per liter of air and are given 
in Table II. 

Passy 's determinations indicate that the lower alco- 
hols have relatively faint odors, but that the higher 

Table II. 

Minimum concentrations for olfaction, in millionths of a gram of alcohol 

per liter of air (Passy, 1892e). 

Alcohol Primary Secondary Tertiary 

Methyl 1000 

Ethyl 250 

Propyl 10 to 5 40 

Normal Butyl 1 20 to 10 

Isobutyl 1 

Normal Amyl 40 to 20 

Active sinistral Amyl 0.6 

Inactive Isoamyl 0.1 

Caprylic 0.005 

members of the series are fairly comparable with, for 
instance, the essential oils. A determination for ethyl 
alcohol by Parker and Stabler in 1913 showed that this 
alcohol could be detected only to a concentration of about 
5.75 milligrams of alcohol per liter of air. The smaller 
amount found by Passy, namely 0.25 milligrams per liter 
of air, is believed by these authors to be due to odorous 
impurities that were found by them in certain ethyl 
alcohols and that may have been present in those tested 
by Passy. 

Some of the more striking determinations by Allison 
and Katz (1919) are reproduced in Table III. 

Here it will be noted that the most active mercaptan 
tested, propyl mercaptan, is detectable at a concentra- 



56 



SMELL, TASTE, ALLIED SENSES 



tion of 0.006 milligrams per liter of air which is equal 
to 6/1,000,000 of a milligram per cubic centimeter. 
This determination is by no means so extreme as that 
of Fischer and Penzoldt, 1/23,000,000,000 of a milligram 
per cubic centimeter. Whether this difference is due to a 
difference in the compounds used, for Fischer and 

Table III. 
Concentrations in Milligrams of Chemical per liter of air. 



Chemical 


Intensity of Odor 


Detectable 


Faint 


Noticeable 


Strong 


Very Strong 


Ethyl ether 


5.833 
3.300 
0.686 
0.046 
0.032 
0.024 
0.018 
0.015 
0.009 
0.008 
0.006 
0.001 
0.00004 


10.167 
6.800 
1.224 
0.088 
0.146 
0.032 

0.039 
0.021 
0.012 
0.020 
0.007 


14.944 
12.733 
2.219 
0.186 
0.301 
0.109 

0.067 
0.066 
0.024 
0.028 
0.011 


17.667 
28.833 
4.457 
0.357 
2.265 
0.332 

0.108 
0.329 
0.030 
0.043 
0.012 


60.600 
46.666 
6.733 
0.501 
5.710 
0.348 

0.144 

0.580 
0.201 
0.054 
0.015 


Chloroform 


Ethyl acetate 


Ethyl mercaptan . . 


Pvridine 


Oil of peppermint 


lodoform 


Methyl isothiocyanate . . . 
Butyric acid 


Allyl isothiocyanate 


Propyl mercaptan 


Amyl thioether 


Artificial musk 





Penzoldt do not state what mercaptan they tested, or 
whether it represents a difference in the methods em- 
ployed cannot be stated. In the table from the work of 
Allison and Katz, as in all previous sets of determination, 
artificial musk is shown to be without question the most 
stimulating substance tested and thus stands at the head 
of olfactory stimuli. 

Notwithstanding the numerous discrepancies between 
the various sets of determinations for olfactory acuity 
made by various workers, it must be admitted that olfac- 



PHYSIOLOGY OF OLFACTION 57 

tion is accomplished through very small, often infinitesi- 
mally small, amounts of material, and yet these amounts 
involve immensely large numbers of molecules of the 
odorous substance. 

4. Physical Condition of Stimulus, Gas or Solution! 
In olfaction in the air-inhabiting vertebrates the stimu- 
lus has been generally assumed to be material particles 
in a vaporous or gaseous condition and not, for instance, 
in the form of a solution. 

This opinion was long ago supported by the experi- 
ments of Tourtual (1827) and especially of Weber (1847) 
both of whom believed that it could be shown that sub- 
stances that could be smelled as vapors could not be 
smelled as solutions when introduced as such into the nose. 
Thus Weber was unable to recognize cologne water when 
this liquid, much diluted with ordinary water, was poured 
into his nasal cavities. He, therefore, concluded that 
though the vapor from cologne water was easily smelled, 
a solution of it was not so sensed and that hence the 
vaporous state of the substance was necessary as a stim- 
ulus for the olfactory organ. This conclusion was ac- 
cepted by a number of investigators including Nagel 
(1894, 1904), Zwaardemaker (1895), Haycraft (1900) 
and others. 

Aronsohn, in 1884, pointed out the great influence 
that water and temperature had on the olfactory organ. 
Ordinary cold water when introduced into the nose will 
so affect the organ of smell that olfaction is impossible 
for some time to come. Cold water is known to excite 
an increased production of mucous whose volume would 
materially interfere with stimulation by covering up the 
olfactory surfaces. Moreover if the action of water on 



58 SMELL, TASTE, ALLIED SENSES 

the organ of smell in an air-inhabiting vertebrate is 
continued for some time, it is said to result ultimately 
in the destruction of the olfactory hairs. Thus Schultze 
(1862) noted that when the olfactory membrane of an air- 
inhabiting amphibian is flooded with water, the cilia with 
which it is provided may continue to beat for hours, 
but the much longer and heavier olfactory hairs vanish 
almost at once. To minimize this deleterious effect Aron- 
sohn, therefore, introduced into the nose material dis- 
solved, not in ordinary water, but in physiological salt 
solution and at an appropriate temperature. With these 
precautions he claimed that it was very easy to recognize 
weak solutions of clove oil. Vaschide in 1901 confirmed 
Aronsohn's results and pointed out that temperature 
was a more important factor in carrying out conclusive 
tests than the composition of the solvent. 

These results, which were in direct opposition to those 
of Weber, were criticized by Zwaardemaker (1895) and 
especially by Veress (1903) who showed that the pro- 
cedure employed by Aronsohn probably resulted in a 
failure to fill the olfactory cleft. Veress maintained that 
unless great care was taken at this step, air was very 
likely to remain in this cleft and thus the solution that 
was being tested would never really reach the olfactory 
terminals. Under such circumstances odorous particles 
would escape from the solution into the air filling the cleft 
and thus reach the olfactory organ as in ordinary olfac- 
tion. Thus it became necessary in making a conclusive 
test to take steps to insure the complete filling of the 
olfactory cleft with the solution to be tested. After 
some experimentation on the human cadaver, Veress per- 
fected a technique whereby this could be accomplished. 



PHYSIOLOGY OF OLFACTION 59 

On thus introducing odorous solutions into the nasal 
chambers of a living subject, he found that these solutions 
were stimuli for the olfactory organs, but that they did 
not produce the sensation ordinarily associated with 
them. A person, however, could soon learn to associate 
a given sensation with a particular substance and could 
thus acquire an ability to recognize this substance, but not 
by what would be called its proper odor. Veress, there- 
fore, concluded that though solutions of odorous materials 
are stimuli for the olfactory organs, they are inadequate 
rather than adequate stimuli. It thus appears, contrary 
to the results obtained by Weber, that the olfactory 
organs of an air-inhabiting vertebrate can be stimulated 
by ordinary solutions, though this form of stimulation 
cannot be looked upon as normal. 

To deny that the olfactory organs of man and other 
like vertebrates are stimulated by solutions, as has been 
done by a number of workers, implies a certain lack of 
appreciation of the actual environment of the olfactory 
terminals. These are the olfactory hairs that project in- 
to the coating of mucous that covers the olfactory mem- 
brane. These hairs appear to be completely covered by 
the mucus and should any of their lash-like ends reach to 
the outer surface of this layer, they are certainly far too 
delicate to project into the adjacent air; they would 
unquestionably remain within the limits of the mucous 
layer. Thus the olfactory hairs are at all times sur- 
rounded by watery mucous, which is in contact on its outer 
face with the air carrying the odorous particles. These 
particles, as already indicated, must be caught in great 
numbers on the moist mucous surface, absorbed according 
to Zwaardeniaker (1918b), and, since they are in the form 



60 SMELL, TASTE, ALLIED SENSES 

of gaseous or vaporous particles, they probably enter 
quickly into solution in the watery mucous and in this state 
come in contact with the olfactory hairs. From the nature 
of the surroundings, then, it would seem extremely im- 
probable that the stimulating material for the olfactory 
terminals should be in any other state than that of a solu- 
tion. This opinion seems to be gaining ground rapidly 
among the more recent workers, for it has found clear 
expression within the last few years in papers by Back- 
man (1917a), by Durand (1918b), and in a qualified way 
by Henning (1916). 

As already indicated, the difficulty met with in at- 
tempting to stimulate adequately the human olfactory 
epithelium with solutions of odorous material is due in 
all probability to the effects of the solvent on the olfac- 
tory hairs and not to the incapacity of these terminals to 
be stimulated by solutions. These hairs are apparently 
very delicately attuned to a mucous environment that 
would be very difficult to duplicate experimentally and yet 
this environment seems to be essential to a wholly suc- 
cessful test. Care as to temperature and salt contents of 
the solvent, as emphasized by Aronsohn, Vaschide, and 
Veress, are probably only the first steps in this direction. 

The relation of the solubility of a substance to its 
efficiency as an olfactory stimulus has been discussed 
recently by Backman (1917a). This investigator has 
expressed the opinion that not only the aqueous environ- 
ment of the olfactory hairs must be considered but also 
the substance of the hairs themselves. This he believes 
to be lipoid in character, an opinion that is supported by 
the well known fact that these hairs are best demon- 
strated by osmic acid. If the embedding mucous layer is 



PHYSIOLOGY OF OLFACTION 61 

watery and the olfactory hairs oily, it follows that any 
substance that gains entrance into the body of the hair 
must first have been dissolved in water and then in oil. 
From this standpoint Backman attempted to determine 
whether there was any relation between the effectiveness 
of certain olfactory stimuli and their solubility in water 
and in oil. Water and olive oil, each at 30 degrees centi- 
grade, were used as the test solvents. Thus methyl alco- 
hol and ethyl alcohol, which are without strong odor, 
were found to be freely soluble in water, but only very 
slightly soluble in oil. Hence while they would dissolve 
abundantly in the olfactory mucous, they would fail to 
enter the hairs to any great extent in consequence of which 
their effectiveness as stimuli must be, according to Back- 
man, very slight. On the other hand normal butyl alco- 
hol has a strong odor and its efficiency as a stimulus was 
believed to depend upon the fact that it is soluble in water 
to the extent of 8.3 per cent and in oil to an almost indefi- 
nite amount. Other substances showed somewhat different 
relations. Thus chlorbenzol could be detected at a dilu- 
tion of 6.7 x 10."* gram-molecules per liter of air, and is 
soluble in water to the extent of 0.25 per cent and in oil 
indefinitely. Brombenzol could be smelled at the some- 
what greater dilution of 1.1 x 10.~ 8 gram-molecules per 
liter of air; yet it is less soluble in water (0.045 per cent) 
than chlorbenzol though indefinitely soluble in oil. In 
these instances the degrees of solubility in water are the 
reverse of the effectiveness of these two substances as 
olfactory stimuli. Possibly solubility in oil, as intimated 
by Larguier des Bancels (1912), is of much more signifi- 
cance for olfactory stimulation than solubility in water. 
If the olfactory hairs in man are provided with flagella, 



62 SMELL, TASTE, ALLIED SENSES 

such as have been described by Jagodowski (1901) in the 
pike, and the distal ends of these flagella reach through 
the olfactory mucous to the nasal atmosphere, the odor- 
ous particles may come directly in contact with them 
and dissolve in their lipoid substance without pass- 
ing through an intermediate watery layer. In that case 
solubility in lipoid would be the only form of solubility 
necessary for the introduction of an effective stimulus. 
That a number of odorous substances are more soluble 
in lipoid than in water has recently been shown by Kremer 
(1917) who found that larger quantities of citral. 
guaiacol, pyridine, and even chloroform and ether would 
dissolve in a saturated aqueous solution of lecithin than 
in pure water. Of course the varying capacity for re- 
action of such materials as may thus become dissolved 
in the substance of the hairs must profoundly influence 
stimulation and possibly it is in this direction that the 
difference between such substances as chlorbenzol and 
brombenzol is to be explained. But however these de- 
tails may be worked out eventually, the general opinion 
that olfactory stimulation is dependent upon some form 
of solution seems to be beyond question. 

That the material thus dissolved must act chemically 
on the olfactory receptors and not by means of any radia- 
tion that it may give out seems probable from the fact 
that olfactory stimuli are substances that are not known 
to be radio-active. That there is a kind of physiological 
radio-activity, such as has been claimed recently for po- 
tassium by Zwaardemaker (1918a, 1920) and as might be 
urged for the materials of olfactory stimulation, seems 
extremely improbable from the recent work of R. F. Loeb 
(1920) and of J. Loeb (1920). Moreover it would be 



PHYSIOLOGY OF OLFACTION 63 

very difficult to explain the variety of olfactory sensa- 
tions on the basis of stimulation by radio-activity, but 
the assumption that the stimuating materials act chemi- 
cally on the substance of the receptor is in easy accord 
with the diversity of olfactory experience. 

5. Olfaction in Fishes. It has already been pointed 
out that most fishes possess paired olfactory sacs whose 
structure and innervation are essentially indentical with 
the corresponding parts in the air-inhabiting vertebrates. 
Nevertheless currents of water flow through these sacs 
and such stimulation as they receive must come from 
these currents. Nagel (1894), who was one of the most 
vigorous opponents to the idea that the olfactory organs 
were stimulated by solutions and believed that gases or 
vapors were the only real stimuli for these receptors, 
was led to conclude that the so-called olfactory organs of 
fishes were fundamentally different from those of the 
air-inhabiting forms and that they probably more nearly 
resembled organs of taste than any other receptor pos- 
sessed by the higher animals. This opinion was based 
upon theoretic considerations rather than upon any par- 
ticular observation or test. 

But before these views had been expressed by Nagel, 
a certain amount of experimental evidence concerning 
olfaction in fishes had been gathered. This was prelimi- 
nary in character and inconclusive, but it nevertheless 
paved the way for further advance. Thus the observa- 
tion of Aronsohn (1884a), that goldfish, which ordinarily 
will eat ant pupae with avidity, would not take these pupae 
after they had been smeared with a little oil of cloves, 
is not final evidence that the fish scented the oil, for 
it is entirely possible that this oil irritated the skin of the 



64 SMELL, TASTE, ALLIED SENSES 

fish's snout and did not stimulate the olfactory apparatus 
at all. Nor was the discoveiy made by Steiner (1888), 
that the spontaneous appropriation of food by the shark, 
Scyllium, ceases on the removal of the cerebral lobes or 
simply on cutting the connection between these lobes and 
the olfactory bulbs, satisfactory evidence that the olfac- 
tory apparatus in these fishes is an organ of smell rather 
than a receptor for taste or some closely allied sense. 
Nagel (1894) noted that the front of the head of the fish, 
Barbus, was as sensitive to sapid substances after the 
olfactory tracts had been cut as before that operation, and 
Sheldon (1909), who studied the dogfish with great ful- 
ness, demonstrated that the decided sensitiveness of the 
nostrils of this fish to weak solutions of oil of cloves, 
pennyroyal, thyme, and the like, was not influenced by 
severing the olfactory crura, but disappeared on cutting 
the combined maxillary and mandibular branches of the 
trigeminal nerve. Evidently the nasal surfaces of fishes 
like those of the higher vertebrates, are innervated by 
fibers from the trigeminal nerve, and it is this nervous 
mechanism rather than the true olfactory apparatus, that 
is stimulated by the substances that have ordinarily been 
applied by experimenters. In 1909, Baglioni showed that 
blinded fishes were excited by the presence of food. None 
of these experiments, however, demonstrated conclu- 
sively that smell rather than taste or some other allied 
sense, was concerned as the receptor. 

As early as 1895 von Uexkull observed that dogfishes 
from which the olfactory membranes had been removed 
did not respond to the presence of food whereas normal 
dogfishes three to five minutes after food had been in- 
troduced into their tank, sought it with great eagerness. 



PHYSIOLOGY OF OLFACTION 65 

In these experiments no attempts were made to exclude 
sight or to ascertain the effects of the operation. In ex- 
periments carried out by me in 1910 an attempt was 
made to gain more conclusive evidence. Five normal 
catfishes (Amiurus) were allowed to swim in an aqua- 
rium in which were hung two wads of cheese cloth one 
containing concealed earthworms, and the other made 
of cloth only. In the course of an hour the wad con- 
taining the worms was seized eleven times by the fishes 
notwithstanding the fact that from time to time this wad 
was interchanged in position with the other. During the 
same period the wad without worms was passed over by 
the fishes many times and never excited any noticeable 
reaction. 

Ten catfishes were next prepared for further experi- 
mentation; in five of these the olfactory tracts were cut 
and from the remaining five the barbels, the seat of the 
chief external gustatory organs, were removed. After 
the fishes had recovered from these operations, they were 
put in an aquarium into which was introduced a wad of 
cheescloth containing minced earthworms. During the 
first hour the wad was seized 34 times by fishes without 
barbels but with normal olfactory organs and, though 
often passed over by fishes with cut olfactory tracts, it 
was never seized by any of these and " nosed" only once 
by one of them. None of these fishes paid any attention 
to a wad of cloth containing no worms. Repetitions of 
these tests gave uniformly similar results and led to the 
conclusion that the olfactory organs of the catfish are 
serviceable in sensing food at a distance much beyond 
that at which the organs of taste are capable of acting; 
in other words, catfishes truly scent their food. 



66 SMELL, TASTE, ALLIED SENSES 

Similar experiments on the killifish (Fundulus) gave 
like results Parker (1911). Here, however, the olfac- 
tory organs were excluded, not by cutting the olfactory 
tracts, but by stitching up the anterior nares. As a re- 
sult of this operation the fish no longer responded to hid- 
den food, but quickly reacquired this power after the 




Fia. 15. Ventral view of the head of a Hammer-head Shark (Cestracion) showing the 
olfactory pita (o) widely separated. After Carman, 1913, Plate 1, Fig. 2. 

anterior nares had been reopened. These results were 
confirmed in work on the dogfish, (Mustelus), by Sheldon 
(1911) and on the swellfish, (Spheroides), by Copeland 
(1912). Sheldon closed the nares of the dogfish with 
cotton plugs and, in 1914, I showed that when only one 
nostril is thus plugged, the fishes turn persistently to- 
ward the side of the open nostril. Such responses 
indicate that in the seeking of food under normal con- 
ditions, dogfishes, and probably other fishes as well, turn 



PHYSIOLOGY OF OLFACTION 67 

toward the side on which the concentration of odorous 
particles is greater. The certainty of this operation 
would increase in proportion as the nostrils of a given 
fish are separated one from the other laterally. A good 
example of an animal in which this condition reaches 
its maximum is seen in the hammerhead shark in which 
the nostrils, as well as the eyes, are carried on the re- 

Table IV. 

Records in per cent, of Turning Movements of three Dogfish under the follow- 
ing successive Conditions: Normal, Left Nostril Occluded, Right Nostril 
Occluded, Both Nostrils Open Parker, (1914). 



States of Fishes 


Turning Movements in Per Cents. 


To left 


To right 


Normal 


50 
11 

87 
44 


50 
89 
13 
56 


Left nostril occluded 


Right nostril occluded 


Both nostrils open 





markable lateral projections that extend sidewise from 
its head into the sea (Fig. 15). 

The turning response of dogfishes under the condi- 
tions mentioned in the preceding paragraph has a 
striking resemblance toi the circus movements in the 
tropic reactions of many of the lower animals and, were 
it not that fishes are so highly organized, it might be 
accepted at once as a response of that kind. The detailed 
condition of such reactions is well illustrated by the 
records in Table IV. 

As a result of the evidence thus far accumulated, it 
seems quite clear, contrary to the opinions expressed by 
Nagel and others, that many fishes scent their food much 
as air-inhabiting animals do and that they must be re- 



68 SMELL, TASTE, ALLIED SENSES 

garded as possessing powers of olfaction fairly compar- 
able with those of the higher forms. This opinion is in 
entire harmony with the well known fact that fishes, 
especially sharks, can be drawn from a long distance by 
ill smelling bait or by oily fish carcasses ground up and 
thrown into the water as in the practice of chumming. 
The extremely small amount of substance needed in these 
operations agrees well with what is known of olfaction 
among air-inhabiting vertebrates and reaches almost in- 
finitesimal proportions as is indicated by the work of 
Olmstcd (1918) on Amiurus. 

The water-inhabiting stages of amphibians will doubt- 
less be found to exhibit the same type of olfaction as 
that seen in fishes. This is already clearly indicated by 
the work of Copeland (1913) on the newt Diemyctylus 
and of Kisser (1914) on tadpoles. 

The opinion that fishes possess powers of olfaction 
comparable with those of the air-inhabiting vertebrates, 
though rejected by many of the older writers, has been 
accepted in recent years by Baglioni (1913) and by 
Luciani (1917). In fishes there can be no doubt that tho 
stimulating material for the olfactory organs is carried 
in the current of water that is passing more or less con- 
tinuously through these parts. Since in air-inhabiting 
vertebrates the stimulating materials are caught on the 
watery mucous of the olfactory surfaces, it follows that, 
as Durand (1918b) has recently declared, the olfactory 
stimulus throughout the whole range of vertebrates is 
material in a state of solution and not simply a gas or 
a vapor. This conclusion is in agreement with the 
opinion expressed many years ago by Johannes Mu'ller. 
Henning (1916), some time since called attention to the 



PHYSIOLOGY OF OLFACTION 69 

possibility that odorous material may form with the ol- 
factory mucous an emulsion rather than a true solution, 
but this suggestion did not seem even to Henning to be 
of much significance, for in other parts of his work he 
refers repeatedly to the state of the stimulating material 
as that of a solution and there appears to be no good 
ground for assuming that such is not the case. 

6. Fatigue and Exhaustion. It is well known that 
the olfactory organs in man are quickly and easily fa- 
tigued by continuous exposure to odorous materials. 
Persons whose occupations lead them to work among 
disagreeable odors soon become insensitive to these and 
it has long been recognized that invalids are not affected 
by the malodors that may come from their own bodies. 
Although these conditions of irresponsiveness may be 
due in part to central nervous states such as lack of at- 
tention and the like, they are also dependent in part 
on peripheral exhaustion. The effects of unpleasant 
smells on the growth of guinea pigs has been tested very 
recently by Winslow and Greenberg (1918). These in- 
vestigators employed a pair of air-proof cages through 
which were passed 1.5 cubic feet of air per minute 
amounting to 4 liters of air per minute for each animal 
in the test. Through one of these cages pure air was 
circulated; through the other, air that had passed over 
fresh moist faeces and that in consequence was impreg- 
nated with a strong faecal odor. A total of 15 sets of 
growing guinea pigs, including 261 animals, were sub- 
jected to these conditions. In the first week of the tests 
the animals supplied with faecal air did not grow as much 
as the controls did, but in the second week they caught 
up in weight with the controls and were thereafter in- 



70 SMELL, TASTE, ALLIED SENSES 

distinguishable from them. Thus the guinea pig, like 
man, though sensitive to disagreeable odors in the be- 
ginning, appears to become in the course of time entirely 
inert to this form of stimulation. 

To test the immediate effects of the continuous action 
of odorous substance on the olfactory organ of man, 
Aronsohn (1884a) determined the length of time certain 
olfactory stimuli at full strength continued to call forth 
sensation. Thus oil of lemon and oil of orange were 
smelled by nine persons till the odors of these substances 
could no longer be perceived The period necessary to 
bring about this obliteration of sensation varied from 
2.5 minutes to 11 minutes with an average of 3 minutes. 
A 0.2 per cent solution of cumarin in water was smelled 
for from 1.75 to 2.3 minutes after which it was no longer 
sensed. Thus olfactory exhaustion under strong stimu- 
lation is accomplished in a very few minutes. The re- 
covery of excitability is apparently equally rapid and 
may be accomplished in as short a time as from 1 to 3 
minutes though complete recovery probably requires a 
longer time. 

Zwaardemaker (1895) tested fatigue in another way 
and determined by means of his olfactometer the in- 
crease in minimum stimulation as the olfactory organ 
gradually approximated exhaustion. During a continu- 
ous stimulation of known intensity the minimum stimu- 
lus was from time to time determined and was found to 
increase steadily. Two substances, benzoin and rubber, 
at two different strengths were tested (Fig. 16). Ben- 
zoin induced fatigue more rapidly than rubber and of 
the two concentrations employed for each substance the 



PHYSIOLOGY OF OLFACTION 



71 



stronger in each instance called forth fatigue more 
quickly than the weaker. 

Some persons are absolutely devoid of true olfaction, 
a condition which, as already pointed out, is attendant 
upon certain deficiencies in the essentials of the olfac- 
tory apparatus and which is designated as anosmia. 



'Olfacties 

n\ 




10 20 30 40 50 60 70 80 90 Sec* 

FIG. 16. Curves of olfactory exhaustion produced by the action of benzoin of 9 and 
of 3.5 olfacties and &by rubber of 14 and of 10 olfacties, acting for different periods. 
The threshold values in olfacties are marked on the ordinates and the duration of 
stimulation in seconds on the abscisste. After Zwaardemaker, 1895, Fig. 22. 

This state may be congenital or acquired and acquired 
anosmia may be either permanent or temporary. Some 
forms of anO'Smia, like color-blindness, are probably in- 
heritable Glaser (1918). Of considerable interest from 
a theoretical standpoint are the cases of partial olfac- 
tory defects. Winkler noted a patient who was quite 
incapable of smelling benzoin though he easily recognized 
musk and another who was just the reverse of the first 



72 SMELL, TASTE, ALLIED SENSES 

one. Blakeslee (1918) has recorded similar cases in 
relation to the odor of verbena flowers. Probably many 
persons are defective in this respect though their defects 
may not have been serious enough to have attracted 
attention and record. 

Temporary partial anosmia may accompany certain 
diseases or may be induced by the application to the 
olfactory surfaces of anesthetizing drugs. Cocaine has 
been used in this way by a number of investigators, in- 
cluding Zwaardemaker, but without very clearly defined 
results. Zwaardemaker observed that temporary anos- 
mia induced in this way was preceded by a brief period 
of increased sensibility or hyperosmia. Subsequently 
Keuter (1900) found that cocaine was also followed by 
hyperosmia. Roljett (1899) produced a complete an- 
osmia by the use of gymnemic acid after which different 
olfactory sensa.tions returned at different intervals. 

7. Qualities of Odors. The qualities of odors ap- 
pear to be almost innumerable. When we attempt to 
name on odor, we almost always designate it by the body 
from which the odorous material emanates like the smell 
of heliotrope, of onion, of rubber, and so forth. With 
tastes, as we shall see later, there are at least four 
clearly marked qualities, sweet, sour, bitter, and salty. 
The first three of these are general terms connected in 
no necessary way with the substances associated with 
them as stimuli, and we are continually finding new sub- 
stances whose tastes are some one of these three. The 
odors of new substances, on the other hand, are almost 
certain to be individual and novel and to agree with odors 
already known only in a most general way. Thus odors 
have a certain historical value and get their names after 
the introduction of the substances with which they are 



PHYSIOLOGY OF OLFACTION 73 

associated; the smell of illuminating gas was not a gen- 
erally known odor till this material was brought into 
common use. Should it be abandoned commercially, its 
odor would cease to be a part of common human sensa- 
tion. In consequence of economic changes many odors 
of trade articles, of kitchen products, and the like have 
disappeared from the list of human sensations and many 
new ones have come in. Yet notwithstanding this rela- 
tively rapid evolution in the field of olfaction, the organ 
of smell seems to remain the same ; it gives up old forms 
of stimulation and takes on new ones in a way that is 
almost incredible. As a result of these peculiarities of 
the olfactory organ the classification of odors has 
proved to be a most perplexing problem and has 
resulted in most instances in what seem to be extremely 
artificial schemes. 

Haller and particularly Linnaeus proposed systems of 
odors that have formed the bases for many of the modern 
classifications such as the one given by Zwaardemaker 
(1895). In this odors are arranged in nine general 
classes each of which may contain two or more sub- 
divisions. These nine classes are briefly as follows : 

1. Etherial odors; three subdivisions: odors of fruits, 

beeswax, ethers. 

2. Aromatic odors ; five subdivisions : odors of camphor, 

cloves, lavender, lemon, bitter almond. 

3. Balsamic odors ; three subdivisions : odors of flowers, 

violet, vanilla, cumarin. 

4. Ambrosial odors ; two subdivisions : odors of am- 

ber, musk. 

5. Alliaceous odors ; three subdivisions : odors of hydro- 

gen sulphide, hydrogen arsenide, chlorine. 



74 SMELL, TASTE, ALLIED SENSES 

6. Empyreumatic odors; two subdivisions: odors of 

roast coffee, benzole. 

7. Caprilic odors; two subdivisions: odors of cheese, 

rancid fat. 

8. Repulsive odors; two subdivisions: odors of deadly 

nightshade, bedbug. 

9. Nauseating odors; two subdivisions: odors of car- 

rion, faBces. 

A survey of this classification shows at once that more 
or less of it is associative and subjective and hence ar- 
tificial, for what may be repulsive to one person may be 
just the reverse to another. It is, therefore, not sur- 
prising that some of the recent students of this subject, 
as for instance Henning (1916), have advised the com- 
plete abandonment of such arrangements and have 
sought to establish by a thorough re-testing of odors an 
impersonal and reasonable classification. As the re- 
sult of an extended and judicious re-examination of odors 
Henning has come to the conclusion that they fell into 
six fundamental classes as follows : 

1. Spicy odors, such as those of fennel, sassafras oil, 

anise, and cloves. 

2. Flowery odors, such as those of heliotrope, cumarin, 

and geranium oil. 

3. Fruity odors, such as those of oil of orange, citro- 

nella, oil of bergamot, and acetic ether. 

4. Resinous or balsamic odors, such as those of tur- 

pentine, of Canada balsam, and of eucalyptus oil. 

5. Burnt odors, such as those of tar and pyridine. 

6. Foul odors, such as those of carbon bisulphide and 

of hydrogen sulphide. 
Although each of these six classes, according to 



PHYSIOLOGY OF OLFACTION 



75 



Henning, is represented by a number of odors, it is not ab- 
solutely separated from the others, but between any pair 
of them there are numerous odors that assume interme- 
diate positions. The six classes, however, are the striking 
predominant elements in this complex and are in no sense 
submerged in the general array of odors. 

Henning has tried to make clear his idea of the rela- 
tions of these six classes by imagining them located one 



flowery 



spicy 




fruity 



\ 

"S 



resinous 



FIQ. 17. Olfactory prism. After Henning, 1916, Fig. 4. 

at each corner of a three-sided prism which he calls the 
olfactory prism (Fig. 17). From each corner of this 
prism lines may be imagined to pass out to the other 
corners ; these lines traverse either the edges of the prism 
or pass over its faces and mark the positions of all in- 
termediate odors. Thus all odors, be they fundamental 
or intermediate, find places on the surface of the prism. 
Relations indicated by lines within the prism and con- 
necting any two points on its surface indicate only 
mixed odors. Thus by means of a figure of three dimen- 
sions Henning brings into clear view the relations 
he conceives to exist between the six fundamental odors, 



76 SMELL, TASTE, ALLIED SENSES 

their intermediates and mixtures. So far as an arrange- 
ment of odors is concerned the clarity of Henning's 
scheme is at once its most attractive and most suspi- 
cious feature. 

8. Chemical Relations of Odors. The scientific 
value of any classification of odors will depend upon the 
success with which such a classification brings the odor- 
ous substances as stimuli into relation with the receptor. 
A satisfactory classification ought to make evident the 
number of elements or components concerned in olfac- 
tion. That olfaction is made up of a number of compo- 
nents is far from established, but what may be called 
the component theory of olfacton is generally assumed 
by the majority of writers on this subject Zwaardemaker 
(1895). That the classification outlined by Zwaarde- 
maker shows very little of this feature is readily admit- 
ted even by this author himself. Quite aside from the 
fact that it may include irritants as well as true odorous 
substances, its classes do not stand up well under experi- 
mental test. Nagel (1897) tested this question in an 
investigation of the odors of vanillin and cumarin. 
These two substances, according to Zwaardemaker 's 
classification, belong not only to the same class of bal- 
samic odors but to the same subdivision, the vanilla odor. 
They ought, therefore, to show considerable olfactory 
similarity. Nagel attempted to test this relationship by 
ascertaining whether the temporary exhaustion of the ol- 
factory organ by one of these substances would influence 
its receptive capacity for the other. To carry out this 
he prepared an aqueous solution of the two substances 
in such proportions that the smell of only vanillin could 
be recognized. He then exhausted the olfactory organ 



PHYSIOLOGY OF OLFACTION 77 

for vanillin by smelling for a long time a pure solution 
of this material. On testing now the solution containing 
the mixture of substances, it was found to smell only 
of cumarin. Thus the exhaustion of the olfactory sur- 
face for vanillin did not prevent stimulation by cumarin. 
The placing of these two substances in the same subdi- 
vison is, therefore, obviously artificial. 

Similar evidence as to the artificiality of Zwaarde- 
maker's classifications had also been obtained from the 
study of persons suffering from partial anosmia and from 
neither this line of investigation nor from that dealing 
with partial exhaustion has there come any special jus- 
tification of the conventional olfactory groupings. 

Yet it is admitted on all sides that olfaction is essen- 
tially a chemical process. And, as a matter of fact, 
some progress has been made in discovering relations 
between chemical structure and olfactory sensation. 
This isi not necessarily of a general nature, but seems/ 
usually to be limited to narrow ranges. Thus among the 
alcohols Passy (1892c) has discovered that the olfactory 
potency increases progressively in passing over this se- 
ries from methyl to amyl as shown in Table V. 

Ba,ckman (1917c) has likewise determined that ini 
the methylbenzene series olfactory acuity for benzene, 
toluene, xylene, cumene, and durene increases as the sub- 
stitute methyl group increases. 

Changes in the quality of odors also follow some natu- 
ral series of organic compounds as has been pointed out 
by Hay craft (1900) in the following etherial salts. 
Ethyl acetate with acetic etherial odor. 
Propyl acetate with acetic odor and slight flavor. 
Butyl acetate with slight acetic odor and pineapple flavor. 



78 SMELL, TASTE, ALLIED SENSES 

Amyl acetate with no acetic odor but well marked pine- 
apple flavor. 

Ethyl acetate and amyl acetate have entirely distinct 
odors, but when propyl acetate and butyl acetate are 
taken into consideration the four compounds form a se- 
ries in which there is a transition in odors corresponding 

Table V. 

X 

Estimated potencies of alcohols, Passy (1892c). 

Alcohol Estimated Potency 

Methyl 1 

Ethyl 4 

Propyl 100 

Butyl 1000 

Amyl 10000 

to the changes in chemical structure. Other series of 
homologues, however, such as the one tested by Huyer 
(1917), analine, o-, m-, and p-toluidine, xylidine, and cu- 
mioline, show no such relations. 

Not a few investigators have suggested that the odors 
of many substances depend upon the number and ar- 
rangement of certain chemical radicals contained within 
the odorous molecule. Such radicals are commonly 
called osmophoric groups. Perhaps one of the most 
considerable studies of this kind was that carried out by 
Cohn (1904), but without commensurate results. The 
most recent and ambitious of these attempts is by Hen- 
ning (1916) whose classification of odors has already been 
referred to. 

Henning's studies on the relations of odors to chemical 
constitution have to do almost entirely with the aromatic 
compounds, though there is no reason to believe that his 
generalizations, if true, may not be extended eventually 



PHYSIOLOGY OF OLFACTION 79 

t . f 

to the aliphatic series. He abandons the idea that spe- 
cial odors are to be associated with particular osmopho- 
ric groups. In odors these groups are significant, not 
because of the structure they themselves possess, but 
because of the positions they may occupy on the benzene 
ring. Osmophoric groups are such as the hydroxyl, al- 
dehyde, keton, ester, nitro, and nitril groups. None of 
these, however, is associated with a particular odor, but 
any one may be the occasion of odor, if it occupies an 
appropriate place on a benzene ring. The position on the 
ring not the particular radical, according to Henning, is 
the determining factor so far as odor is concerned. 

Henning is further convinced that in a general way 
types of chemical constitution can be indicated for the 
six groups of odors that he was able to distinguish 
(Fig. 18). Thus the class of spicy odors is represented 
by compounds in which the osmophoric groups are in 
para-position (Fig. 18a), as in anisaldehyde. In the 
flowery odors the osmophoric groups are in the meta- or 
the ortho-positions (Fig. 18b), as in tuberon. In the 
fruity odors the groups are forked (Fig. 18c) as in cit- 
ral. In the resinous odors the groups are within the 
ring (Fig. 18d) as in pinene. In the burnt odors the 
ring is smooth (Fig. 18e) as in pyridin, and in the foul 
odors the ring is fragmentary (Fig. 18f ) as in cacodyl. 
In this way each class of odors is associated with a spe- 
cial feature in the constitution of the molecule though 
not necessarily with a particular osmophoric group. In- 
termediate odors are due to combinations of groupings 
which partake of the nature of the two classes between 
which the intermediate lies. Thus vanillin has an odor 
between spicy and flowery and its three osmophoric 



80 SMELL, TASTE, ALLIED SENSES 

groups (Fig. 18g) are attached so as to represent both 
the para-position (spicy) and the ortho-position (flow- 
ery). By this ingenious system Henning has attempted 
to connect odor with chemical constitution and though 









Fio. IS. Generalized diagrams of the molecular structure of the six classes of aromatic 
bodies that serve as olfactory stimuli according to Henning (1910); a, for spicy odors; b. for 
flowery odors; c, for fruity odors; d, for resinous odors; e, for burnt odors; f, for foul odors; 
and g, for au intermediate odor between spicy and flowery. 

the attempt is avowedly fragmentary and may be open 
to much subsequent modification, it gives promise of 
the solution of a problem that heretofore has been 
most baffling. 

9. Inadequate and Adequate Stimuli. Inadequate 
olfactory stimuli are apparently very few in number and 
not well known. Thermal stimuli when applied to the 



PHYSIOLOGY OF OLFACTION 81 

olfactory organs are said to call forth no sensations of 
smell, and Valentin's statement that mechanical stimuli 
will produce unpleasant olfactory sensations has not been 
confirmed. Aronsohn (1884b), after filling the nasal 
cavity with warm physiological salt solution led a direct 
electric current through this cavity with the result that 
certain obscure sensations were produced depending upon 
whether the anode or the cathode was within the nose. 
With the anode in the nose a sensation was called forth 
on opening the circuit; with the cathode in the nose on 
closing it. There was, however, no evidence to show 
that these effects were not due to a stimuation of tri- 
geminal endings instead of olfactory endings. Althaus 
in 1869 recorded as the outcome of electrical stimulation 
a phosphorous-like smell in a patient suffering from 
double trigeminal paralysis. Apparently the electric 
current is a true inadequate stimulus for the olfactory 
organ, but its peculiarities are very incompletely under- 
stood. Aside from this and the effects from solutions 
as described by Veress, inadequate olfactory stimulation 
seems not to exist. 

The adequate olfactory stimulus for both water-in- 
habiting and air-inhabiting vertebrates is a solution in 
contact with the olfactory hairs and perhaps formed in 
part within these bodies. The solvent is probably first 
the olfactory mucous which receives the solute from the 
current of water or of air that passes over its outer sur- 
face. This watery solvent, which from its nature must be 
almost universal in its dissolving power, passes the solute 
on to the olfactory hairs whose capacity as receptors is 
probably limited by their lipoid composition. Only those 
substances that are soluble in lipoids can be taken up by 

6 



82 SMELL, TASTE, ALLIED SENSES 

the hairs, a process that must precede the initiation of the 
olfactory nerve-impulse. The solute may be any one 
of an immense variety of substances whose primary char- 
acteristics are that they are not only soluble in water but 
also in oil. The amount of these substances necessary 
for olfaction even in the case of the least odorous of 
them is very small and in that of the most odorous in- 
credibly small. The amounts that are usually estimated 
for olfaction are those contained in what is believed to 
be the minimum volume of water or of air necessaiy 
for stimulation, but of the very minute amount of odor- 
ous substance contained in this volume only a veiy small 
fraction of it can reach the olfactory hairs. Much must 
be carried away in the general current or left stranded 
on non-olfactory portions of the nasal surfaces. Whether 
the olfactory hairs can concentrate this material or not 
remains to be ascertained, but even assuming that they 
can, the effective concentration must be of an extremely 
low order. 

The substances thus brought in solution into the ol- 
factory hairs must initiate those nervous changes that 
eventually produce the olfactory sensations. There 
ought, therefore, to be some relation between these sub- 
stances and the resulting sensations. It is generally 
assumed that the substances that act as olfactory stimuli 
fall into classes associated with corresponding classes 
of sensations. As already indicated this conception may 
be called the component theory of olfaction, and if we 
assume, for instance, that the six classes of odors dis- 
tinguished by Henning are separate classes, a view that 
Kenning, however, opposes, then these classes would 



PHYSIOLOGY OF OLFACTION 83 

represent the olfactory components that physiologists 
have sought for so long. 

The very existence of partial anosmia implies olfac- 
tory components the inactivity of one of which is ac- 
countable for the partial defect. But such cases are too 
little known to admit of clear interpretation. Thus 
Aronsohn's observation (1886) that partial anosmia pro- 
duced by the exhaustion of the nose through ammonium 
sulphide leaves that organ sensitive to etherial oils but 
insensitive to hydrogen sulphide, hydrochloric acid and 
bromine, may be a differential effect between true odors 
(olfactory endings) and irritants (trigeminal endings), 
and not between groups of true odors. Nevertheless it 
must be in this direction that an experimental analysis of 
the general problem of olfaction will eventually proceed. 

From this standpoint the condition presented by 
mixed odors is of significance. At least two classes of 
odor mixtures are to be distinguished, one spurious and 
the other real. Spurious mixed odors are those in which 
the gases or vapors act chemically on each other and thus 
produce a third substance which may or may not have 
an odor of its own. Thus ammonia and acetic acid both 
stimulate the nose, but when mixed they possess no odor 
for they combine to form odorless ammonium acetate. 
Obviously such instances are not, accurately speaking, 
instances of mixed odors. On the other hand there are 
many pairs of odorous substances in which one member 
does not act upon the other chemically and consequently 
in which the two are left to act independently on the ol- 
factory receptors. Such double stimuli, from the stand- 
point of the component theory might be expected to excite 
two sensations, but apparently this is not always the 



84 SMELL, TASTE, ALLIED SENSES 

case. If in a pair of such odors one is much stronger 
than the other, its smell dominates completely. If, how- 
ever, the two odors are closely balanced a true odor may 
result which in quality is said to be unlike that of either 
component. Novel odors of this kind may be produced, 
according to Aronsohn (1886), by such combinations as 
cologne water and oil of orange, cologne water and oil of 
lemon, oil of bergamot and oil of orange, and so forth. 
The condition that thus produces a noval odor is one of 
considerable delicacy and may be easily upset by the 
greater exhausting effect of one or other of the components 
thus allowing the less exhausted member to assert itself 
and to call forth its own peculiar sensation. The presence 
of a sensation different from those of the pair of stimuli 
producing that sensation, might seem to be a condition 
adverse to the component theory, but it must be remem- 
bered that in vision, in which the component conception 
is fundamental, an exact parallel occurs. Thus when a 
pure orange light is mixed with a pure green light, there 
may result a sensation of yellow that is wholly unlike 
that appropriate to either member of the combination, 
and that, as a matter of fact, may be indistinguishable 
from a sensation of yellow produced by a pure yellow 
] ight. Thus in accepting the component theory of sensory 
activity it must be admitted that two stimuli together 
may excite a receptor in precisely the same way as a 
third and entirely different stimulus may do. The exist- 
ence of a novel olfactory sensation due to the simultane- 
ous activity of two independent stimuli is therefore, no 
serious obstacle to this theory. 

The condition of double olfactory stimulation that 
has just been described must not be confused with a kind 



PHYSIOLOGY OF OLFACTION 85 

of double stimulation that has been much studied. Val- 
entin observed that when ether and balsam of Peru were 
smelled at the same time one by one nostril and the other 
by the other nostril, the odors are perceived not together 
but alternately and Valentin believed that there was 
a sensory conflict here as in vision, when one eye is 
directed to a field of one color and the other eye to one 
of another color. Aronsohn (1886) noted a similar con- 
flict between the smell of camphor and that of oil of 
lemon. He also discovered that under similar circum- 

Table VI. 
Pairs of neutralizing odors (Zwaardemaker, 1895, p. 168). 

Pairs of odorous bodies Neutralizing Strength 

in olfacties 

Cedarwood and rubber 2.75 : 14 

Benzoin and rubber 3.5 : 10 

Paraffin and rubber 8.5 : 14 

Rubber and wux 14 :28 

Rubber and balsam of Tolu 14 :70 

Wax and balsam of Tolu 40 :90 

Paraffin and wax 10 :20 

stances one smell could overcome another. Thus the 
smell of camphor was neutralized by the smell of pe- 
troleum, cologne water, oil of juniper and so forth. 
This question was investigated much more fully by 
Zwaardemaker (1895) who employed for this purpose 
his double olfactometor. By this means it was compara- 
tively easy to- balance odors and then lead one into one 
nasal cavity and the other into the other cavity. In this 
way complete neutralization could be attained with great 
accuracy. Table VI gives a list of neutralizing pairs of 
odors and the intensity in olfacties at which Zwaarde- 
maker found neutralization to occur. 



86 SMELL, TASTE, ALLIED SENSES 

It is needless to say that since in this form of double 
stimulation one stimulus is applied to one olfactory organ 
and the other to the other organ, the phenomenon of neu- 
tralization cannot depend upon the chemical action of 
one odor upon the other, for the odorous materials are 
not allowed to mingle. The fact that they are separately 
applied to different receptors shows that this type of 
conflict and of neutralization must have a central origin. 

10. Olfactory Eeflexes. In discussing the relations 
of the two categories of nasal stimuli, irritants and true 
odors, Frohlich attributed reflex action to the first but 
not to the second, and it is true that nasal irritants 
almost invariably call forth vigorous respiratory re- 
sponses, such as sneezing, whereas true odors are seldom 
followed by reactions of a marked kind. Pawlow, how- 
ever, has pointed out the great importance of true odors 
in exciting and, in a way, in controlling the whole chain 
of digestive secretions, a process just as significantly 
reflex as sneezing but not so easily observed. Both 
classes of stimuli, then, are followed by abundant and 
important reflexes, but in one class these are of a kind 
easily noticed, in the other they are more hidden. 

Although the olfactory organs in man are unques- 
tionably concerned with the odors of the food that is 
being masticated, they are much more concerned with 
the odors of the environment. From this standpoint the 
olfactory organs are properly classed as distance-recep- 
tors or receptors affected by stimuli that emanate from 
more remote points in the surroundings. In consequence 
our olfactory sensations are in a way projected into the 
exterior and we seek, avoid, or recognize the distant body 
by its odor. The smell of a skunk is unquestionably a 



PHYSIOLOGY OF OLFACTION 87 

protective odor in that it implies that it can be sensed 
by other animals that will thereupon avoid its source. 
The great delicacy of olfaction among the higher animals 
by which they can scent the hunter is well known. Other 
odors have much to do with sexual activities whereby one 
sex is led to find the other or is otherwise excited to ac- 
tivity. But the prime service of olfaction is in the quest 
of food. From the fishes to the mammals olfaction 
serves as a means of discovering hidden or remote food 
and in this respect it is a highly significant sense for the 
direction of locomotion. In man and other microsmatic 
forms much of the keenness of olfaction has disappeared 
and yet the high development of this sense in our an- 
cestry has left such a profound impression on the 
organization of our central nervous apparatus that we are 
often surprised by the power of our olfactory associations. 

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88 SMELL, TASTE, ALLIED SENSES 

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BAGLIONI, 8. 1913. Die niederen Sinne. \\'i>itcrstcin, Ifa>ull>. rcri/1. Phys- 
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CHAPTER IV. 

VOMERO-NASAL ORGAN OR ORGAN OF JACOBSON. 

Contents. 1. Vomero-nasal Organ in Man. 2. Com- 
parative Anatony. 3. Histology. 4. Adjacent Parts. 
5. Function. 6. Bibliography. 

VOMERO-NASAL Organ in Man. In early infancy all 
human beings show traces of a pair of organs that are 
without doubt homologues of the vomero-nasal organs of 
the lower vertebrates. Each vomero-nasal organ in the 
new-bora babe is a short tubular structure from a half 
to two and a half millimeters long and lodged in the lower 
anterior portion of the nasal septum. The organ opens 
into the nasal cavity by a minute pore on the free sur- 
face of the septum not far from its ventral border and 
onlv a short distance inward from the external naris. 

/ 

The tubular part of the organ extends posteriorly from 
this minute pore and ends blindly at a point somewhat 
higher than the level of the pore itself. 

In early human embryos the pore of the vomero-nasal 
organ can be easily identified on the median face of the 
nasal chamber just within the anterior naris (Fig. 19). In 
adults the organ, though commonly present, may disap- 
pear completely. When present it occurs near the ven- 
tral margin of the nasal septum (Fig. 20). Kolliker 
(1877) states that it may vary in length from two to 
seven millimeters and Anton (1895) gives as the extremes 
2.2 millimeters and 8.4 millimeters. As seen in trans- 
verse section it has the appearance of a tube flattened in 
the plane of the nasal septum. Its lateral wall is cov- 

92 



VOMERO-NASAL ORGAN 



93 



ered with an epithelium that resembles histologically the 
respiratory epithelium of the nasal cavity. This lateral 
epithelium may even be ciliated. The median wall is cov- 
ered with an epithelium much like the olfactory epithelium 
of the nose except that differentiated olfactory cells are 
apparently not present. The cavity of the organ is 
sometimes obliterated by excessive 
epithelial growth and calcareous con- 
cretions may occur in its walls. As it 
appears to be without nervous con- 
nections, the vomero-nasal organ in 
man is probably entirely rudimentary. 
2. Comparative Anatomy. A vo- 
mero-nasal organ has been recog- 
nized for some time past in all classes 
of vertebrates except the fishes, but, 
according to Gawrilenko (1910), this 
group too must be admitted to have at 
least the f oreshadowings of such an 
organ 

_ 

as the sharks and rays each oltactory 
sac is divided into two compartments 
with separate innervation and these two compartments 
may be supposed to correspond one to the vomero-nasal 
organ and the other to the olfactory organ proper. This 
double character of the olfactory apparatus is also seen in 
other fishes. Thus in the development of the olfactory sac 
of the salmon Gawrilenko has shown that this organ 
includes two sensory thickenings or placodes, a median 
one and a lateral one. These two placodes can be 
traced into the adult where they are said to give rise to 
a median olfactory area and a lateral area. The median 




F IG. 19. Lateral view 

-.-. . , ... ^ of the head of a human 

Even in such primitive iorms embryo showing the pore 

(v) of the vomero-nasal 

After HIS, isss, 



94 



SMELL, TASTE, ALLIED SENSES 



area is believed to correspond to the vomero-nasal organ 
of the higher vertebrates and the lateral area to the true 
olfactory receptor of these forms. 

In some amphibians the distinction between a lateral 
and a median organ is much more evident than in fishes 
(Fig. 21). The lateral organ is the one that conducts the 




Fio. 20. Diagram of the median face of the left nasal ravity of man ; the small circle marks 
the position of the vomero-nasal organ in the nasal septum. 

newly established air current from the external naris to 
the choana and hence corresponds to the olfactory organ 
proper. The median cavity is less involved in this cur- 
rent and is believed to represent the vomero-nasal organ. 
In certain sauropsida such as the alligators and tur- 
tles the vomero-nasal organ has been said to be at best 
only poorly developed, though so far as turtles are con- 
cerned this opinion is not shared by one of the most 
recent workers, McCotter (1917). In birds the organ 
is claimed to be entirely absent, but in lizards and in 
snakes it is highly differentiated (Fig. 22). Here the 
olfactory apparatus consists of a well-developed organ 



VOMERO-NASAL ORGAN 



95 



of smell located in the respiratory passage and an 
entirely independent vomero-nasal organ. The latter, 
in the form of a blind sac, opens into the cavity of the 
mouth. This peculiarity is probably dependent upon the 
growth of the hard palate in reptiles whereby a new 
adjustment between the nasal cavity and the mouth is 
brought about. 

In mammals the vomero-nasal organ also shows much 
diversity. It is apparently best developed in the lower 




Fia. 21. Transverse section of the snout of a young frog snowing the partial division 
of the nasal cavity into a lateral or olfactory portion (o) and 
a median or vomero-nasal portion (v). 

i 

forms, such as the Australian duckbill Ornithorhynchus, 
and it is rudimentary in such groups as the primates 
including man. In general it has the form of a blind 
sac that opens usually by means of the naso-palatine duct 
(Stenson's duct) into the mouth, a relation that is prob- 
ably reminiscent of its original connection with the 
primitive choana of which the naso-palatine duct may 
be regarded as a trace. Less commonly it opens directly 
by its own duct into the nasal cavity. This condition 
obtains in certain rodents such as the rabbit, guinea pig, 
rat, and mouse, and in certain primates including man. 



96 



SMELL, TASTE, ALLIED SENSES 



In all these higher vertebrates the olfactory organ proper 
corresponds to the lateral component of the pair of 
organs in the lower forms and the vomero-nasal organ 
to the median member of this group (Fig. 23). 

3. Histology. The vomero-nasal organ of the dog 
and the cat, as described by Bead (1908), is a tubular 

organ whose transverse 
section is circular in out- 
line near its opening 
and crescentic or kidney- 
shaped throughout its 
greater extent. Its median 
wall may be two to three 
times as thick as its lateral 
wall. This thickened por- 
tion, which has been 
observed by numerous 
workers in a variety of 
mammals, is similar in 

Fin. 22. Transverae section of the head Cellular Composition to the 

of a snake embryo (Agkistrodon) showing , . -JIT c 

the nasal canal proper (n) and the large OltaCtOry epithelium Ol 

vomero-nasal (v) organ opening on the ' . 

roof of the mouth. Preparation by Mr. tll6 11OSC aild IS 111 StrOllg 
F. B. Manning. 

contrast with the lateral 

thin wall which resembles respiratory nasal epithelium. 
Read has shown that the vomero-nasal organ of the cat 
and the dog is like the olfactory region of the nose in that 
it receives nerve fibers from two sources, the olfactory 
nerve and the trigeminal nerve. 

As early as 1892 von Brunn showed that the sense 
cells of the vomero-nasal organ of the sheep were con- 
nected with nerve fibers in exactly the way they were in 
the olfactory region proper and he assumed, probably 




VOMERO-NASAL ORGAN 



97 



with correctness, that these fibers belonged to the olfac- 
tory nerve (Fig. 24). These observations were confirmed 
by all subsequent workers including von Lenhossek (1892) 
in the rabbit, Retzius (1894) in the snake, Ramon y Cajal 
(1895) in the rat, and Read (1908) in the kitten. 
Retzius showed that in the snake those nerve-fibers that 
were connected with the sense cells in the vomero- 
nasal organ mingled with the bundle of fibers from the 
olfactory region of 
the nose and thus con- 
firmed von Brunn's 
suspicion that vo- 
mero-nasalfiberswere 
true olfactory fibers. 

Von Lenhossek 
pointed out that at 
least in the rabbit the 
sense cells were not 
limited to the thick- 
;ened face of the vo- 
inero-nasal organ, as 
had been maintained heretofore, but were found upon the 
opposite tlu'n face of the organ as well This observation 
was confirmed on the rat a few years later by Ramon y 
Cajal. Hairlike terminations on the vomero-nasal sense 
cells, such as those that had been found in the olfactory 
cells, were sought for by a number of investigators 
but only traces of these structures could be found (von 
Brunn, Retzius, Read), probably because of the ease 
with which they are Destroyed in the preparation of 
the tissue. 

Von Lenhossek in 1892 not only confirmed von 

7 




FIG. 23. Transverse section of the nasal 
septum of a young cat showing the vomero- 
nasal organ (v), its.cartilage (c), and the nasal 
cavity (n). 



98 



SMELL, TASTE, ALLIED SENSES 




Brunn's observation that the vomero-nasal sense cells 
were directly connected with nerve fibers, but he also 
pointed out that in the Jacobson organ of the foetal rab- 
bit free-nerve terminations occurred. These free termi- 
nals in some instances reached the receptive surface of 
the epithelium where they ended in slight knobs. Similar 

endings were recorded for the 
rat by Bamon y Cajal (1895). 
Von Lenhossek was unable to 
decide definitely whether these 
terminals belonged to the olfac- 
tory or to the trigeminal nerve. 
Nor is this question definitely 
settled now, though, judging 
from the conditions met with 
in the olfactory organ of 
the nose, it is highly prob- 
able, as Eead concludes, that they belong to the trigem- 
inal nerve. Admitting this to be the case, the innervation 
of the vomero-nasal organ would agree in all particulars 
with that of the olfactory organ proper. It is quite 
clear from the studies of Brookover (1917) on the ner- 
vus termmalis as well as from those of Larsell (1918) 
that the relations of this nerve to the vomero-nasal organ 
are merely incidental; the terminal nerve is in no sense 
especially connected with the organ of Jacobson. 

4. Adjacent Parts. In many of the higher verte- 
brates the vomero-nasal organ is contained within a more 
or less complete capsule of cartilage, the Jacobson car- 
tilage (See Fig. 23). In the cat this capsule, according 
to Read, is complete anteriorly and incomplete posteri- 
orly; in the dog it is incomplete throughout its whole 



Fio. 24. Epithelium from the 
vomero-nasal organ of the sheep 
showing the receptive cells impreg- 
nated by the Golgi method. After 
von Brunn, 1892, Plate 30, Fig. 12. 



VOMERO-NASAL ORGAN 99 

length. The vomero-nasal organ of these forms has com- 
monly associated with it a considerable amount of caver- 
nous tissue. This tissue, which was long ago identified 
in nasal organs by Klein (1881a, 1881b), is so disposed 
that in connection with the surrounding cartillage and 
other parts, it may serve as a means of changing in no 
small degree the volume of the organ. 

5. Function. Concerning the function of the vomero- 
nasal organ almost nothing is known. Von Mihalkovics 
(1898) found that after burning out the naso-palatine 
duct and more or less of the vomero-nasal organ 
in the cat and in the rabbit, the appropriation of food 
by these animals was not interfered with, but it is hardly 
to be expected that so crude an experiment as this would 
yield significant results. Kolliker emphasized the fact 
that, at least in mammals, the connection between the 
vomero-nasal organ and the exterior is so narrow and 
indirect that it seems almost impossible that there should 
be any transfer of material from the exterior to the inte- 
rior of the organ as, for instance, is implied in olfaction. 
He, therefore, suggested that the vomero-nasal organ 
was concerned with testing the animal's own juices as rep- 
resented by the secretions from this organ. But the 
vomero-nasal organ, particularly in mammals, is inti- 
mately associated with much cavernous tissue whose 
change in volume may be concerned with its filling and 
emptying. Thus it is quite possible that oral or nasal 
juices may be sucked into the vomero-nasal organ and 
discharged from it as has recently been maintained by 
Broman (1918). Henning (1916) has suggested that the 
organ is concerned with water olfaction as contrasted 
with air olfaction, but according to an unpublished obser- 



100 SMELL, TASTE, ALLIED SENSES 

vation of Mr. H. E. Hamlin air is often found in the 
vomero-nasal organs of freshly killed mammals, and this 
observation when taken in connection with the work of 
Broman supports the hypothesis already advanced by 
many investigators (P. and F. Sarasin, 1890; Seydel, 
1895; Gaupp, 1900) that these organs are subsidiary 
olfactory receptors, an opinion that, while it lacks com- 
plete experimental proof, is abundantly supported by the 
finer structure of the parts concerned. 

6. BIBLIOGRAPHY. 
ANTON, W. 1895. Beitriige zur Kenntnis dos Ja^obson't-chen Organes bei 

Erwachsenen. Zeitschr. Eeitk., Bd. 16, pp. 355-372. 
BROMAN, I. 1918. Om Jacobsonska Organets konstruktion om funktion. 

Lunds Univ. Arsskrift, N. F., Avd. 2, vol. 14, No. 4, 40 pp. 
BROOKOVER, C. 1917. The Peripheral Distribution of the Nervus termin- 

alis in an infant. Jour. Comp. Neurol., vol. 28, pp. 349-360. 
VON BRUNN, A. 1892. Die Endigung der Olfaetoriusfasern im Jacob- 

son'schen Organe des Schafes. Arch mik. Anat., Bd. 39, pp. 651-652. 
GAUPP, E. 1900. Das Chondrocranium von Lacerta agilis. Anat. Hefte, 

Arb., Bd. 15, pp. 433-595. 
GAWRILENKO, A. 1910. Die Entwickeltmg des Geruchsorgan bei Salmo 

salar. Anat. Anz., Bd. 36, pp. 411-427. 
His, W. 1885. Anatomic menschlicher Embryonen. Ill Zur Geschichtc 

der Organe. Leipzig, 260 pp. 

HENNING, H. 1916. Der Geruch. Leipzig, 533 pp. 
KALLIUS, E. 1905. Geruchsorgan. Bardeleben, Handb. Anat. Menschcn, 

Bd. 5, Abt. 1, Teil 2, pp. 115-242. 
KLEIN, E. 188 la. Contribution to the Minute Anatomy of the Nasal 

Mucous Membrane. Quart. Jour. Mic. Soi., vol. 21, pp. 98-1 13>. 
KLEIN, E. 1881b. A Further Contribution to the Minute Anatomy of 

the Organ of Jacobson in the Guinea-pig. Quart. Jour. Mic. Sri., 

vol. 21, pp. 219-230. 

KOLLIKER, A. 1877. Ueber des Jacobsonsche Organ des Menschen. Graf. 

Kchrift. Rinecker. 
VON LENHOSSEK, M. 1892. Die Nervenurspriinge und -Endigung^n im 

Jacobson 'echen Organ des Kaninchens. Anat. Anz., Bd. 7, pp. 628-635. 
LARSELL, 0. 1918. ' Studies on the Nervus terminalis: Mammals. Jour. 

Comp. Neurol., vol. 30, pp. 1-68. 



VOMERO-NASAL ORGAN 101 

McCoTTER, R. E. 1917. The Vomero-nasal apparatus in Chrysemyg punc- 
tata and Rana catesbiana. Anat. Rec., vol. 13, pp. 51-67. 

VON MIHALKOVICS, V. 1898. Nasenhohle und Jacobsonches Organ. Anat. 
Hefte. Art., Bd. 11, pp. 1-107. 

RAMON Y CAJAL, S. 1895. Les nouvelles Idees sur la Structure du 

Syst&me Nerveux. Paris, '201 pp. 
READ, E. A. 1908. A Contribution to the Knowledge of the Olfactory 

Apparatus in the Dog, Cat, and Man. Amer. Jour. Anat., vol, 8, 

pp. 17-47. 
RETZIUS, G. 1894. Die Riechzellen der Ophidier in der Riechschleiim- 

haut und im Jacobson'schen Organ. Biol. Unters. N. F., Bd. 6, pp. 48-51. 
SABASIN, P. und F. SAEASIN. 1890. Zur Entwicklungsgeschichte Und 

Anatomie der ceylonesischen Blindwiihle Ichthyophis glutinosus L. 

Ergeb. naturw, Forsch. Ceylon, Bd. 2, pp. 1-263. 
SEYDEL, O. 1895. Ueber die Nasenhohie und daa Jacobsonsche Organ 

Der Amphibien. Morph. Jahrb., Bd. 23, pp. 453-543. 
SYMINGTON, J. 1891. On the Nose, the Organ of Jacobson, and the 

Dumb-bell shaped Bone in Ornithorhynchus. Proc. Zool. Soc. London, 

1891, pp. 575-584. 
ZUCKERKANDL, E. 1910. Das Jacobsonsche Organ. Ergeb. Anat. Ent- 

wick., Bd. 18, pp. 801-843. 



CHAPTER V. 

THE COMMON CHEMICAL SENSE. 

Contents. 1. Common Chemical Sense in Man. 2. In 
Lower Vertebrates. 3. Nerve Terminals. 4. Relation 
to Other Senses. 5. Bibliography. 

1. COMMON Chemical Sense in Man. It was long 1 
ago made clear by Frohlich that on the nasal surfaces 
of man there were two systems of receptors that could 
be stimulated by gaseous or vaporous materials: olfac- 
tory cells representing the olfactory nerve, whose stim- 
uli, delicate perfumes and odors, call forth few observable 
responses, and free-nerve terminals probably represent- 
ing the trigeminal nerve, whose stimuli, irritants for the 
most part, are usually followed by vigorous reactions 
such as sneezing. This distinction has been generally 
accepted among physiologists, but it has not been so 
clearly seen that the receptors for irritants are found in 
other parts of the body than the nose and that they rep- 
resent a fairly well denned category of sense organs 
which, if not so sharply marked off as those of taste and 
of smell, are fairly comparable in distinctness with the 
receptors for heat, cold, or pain. The extent of their 
occurrence is easily recognized. Thus the vapor of 
ammonia not only irritates the nose, but also the eye, 
causing watering, as well as the mouth and the upper 
respiratory region whence arise impulses that lead to 
coughing and choking. Irnlnnts of this kind also stim- 
ulate the anus and the genital apertures and in fact any 

102 



THE COMMON CHEMICAL SENSE 103 

part of the body where a mucous surface is in contact 
more or less with the exterior. In man, then, the recep- 
tors for irritants have a much wider distribution over 
the body than the olfactory receptors have in that they 
are found on almost every exposed or partly exposed 
mucous surface. 

2. In Lower Vertebrates. In other mammals than 
man, in birds, and in reptiles the receptors for irritating 
substances are probably distributed in much the same 
way as in man and are confined to the exposed or semi- 
exposed mucous surfaces. In the amphibians and the 
fishes, however, this system of receptors shows a pro- 
digious expansion in that in these animals it is found 
covering their whole exteriors. The well known experi- 
ment of stimulating the frog's foot with solutions of 
acids and other such substances is based upon this peculi- 
arity and the sensitiveness of the skin of this and other 
amphibians and of fishes as worked out by Nagel (1894), 
Parker (1908a, 1908b, 1912), Sheldon (1909), Cole (1910), 
Crozier (1915, 1916), and others show quite clearly that 
sensitiveness to solutions of chemicals is a common 
property of the skin in all these aquatic vertebrates. 

As early as 1894 Nagel discovered that the integument 
of the dogfish Scyllium was extremely sensitive to a great 
variety of chemical substances. He likewise found that the 
skin of the goosefish Lophius and of the lancetfish Amphi- 
oxus were also generally open to chemical stimulation. 

Nagel 's observations on Amphioxus were confirmed 
in 1908 when it was shown that the skin of this fish was 
sensitive to solutions of acids, alkalis, alcohol, ether, 
chloroform, turpentine, oil of bergamot and oil of rose- 
mary but not to solutions of sugar. It was also demon- 



104 SMELL, TASTE, ALLIED SENSES 

strated that the skin of the catfish Amiurus was sensitive 
to sour, saline, and alkaline solutions, a condition that 
was subsequently found to be true for the young of the 
lamprey eel Ammocoetes Parker (1908b, 1912). In 1909 
Sheldon published an account of the chemical stimulation 
of the skin of the dogfish Mustelus, the most extensive 
study of this kind thus far made. Sheldon found that 
the whole outer surface of this fish was very sensitive 
to acids and alkalis, less so to salts and bitter substances 
and not at all to sugar solutions, a condition that in gen- 
eral confirmed the results of earlier workers. Crozier 
(1915) studied the mutual relations of salts of sodium, 
potassium, and calcium as applied to the frog's skin and 
was able to demonstrate ionic antagonism which led him 
to conclude that in normal stimulation the surface of the 
receptor must be penetrated by the stimulant. 

These observations warrant the general conclusion 
that the outer surfaces of most fishes and amphibians are 
open to stimulation by chemical substances of a mildly 
irritating kind. It is probable that this capacity has 
been retained by the air-inhabiting vertebrates in only 
a very circumscribed and local way, namely on those 
exposed or partly exposed mucous surfaces that reproduce 
in their delicacy and moistness the characteristics of the 
general outer surface of aquatic forms. From this 
standpoint the restriction of the chemical sensibility of 
the air-inhabiting vertebrates is the result of the drying 
of their skins in consequence of an ancestral migration 
from an environment of water to one of air. 

3. Nerve Terminals. The form of nerve terminal 
that is concerned with the reception of chemical irritants 
in the skin of vertebrates is well indicated in the catfish 



THE COMMON CHEMICAL SENSE 105 

Amiurus. If a bait in the form of a piece of meat or the 
like is held close to the flank of one of these fishes, the 
animal is very likely to turn suddenly and snap it up. This 
is not a surprising response, for the sides of these ani- 
mals are well provided with taste-buds. They are also 
supplied with lateral-line organs. Both these sets of 
receptors may be eliminated by cutting on the one hand, 
the branch of the facial nerve that is supplied to the taste- 
buds of the side of the body and, on the other, the lateral- 
line nerve that is distributed to the lateral-line organs 
of the same region. After the fish has recovered from 
such an operation, it will no longer respond to a bait held 
near its flank, but the skin of this region is still per- 
fectly open to stimulation by sour, saline and alkaline 
solutions. As the only receptors left after the operation 
just described are the free-nerve terminals of the spinal 
nerves, these terminals must be the receptors for chem- 
ical irritants. This conclusion is in accord with the fact 
that this type of ending is the only one that occurs in 
many portions of the skin of the dogfish, of the foot of 
the frog, and of the partly exposed mucous surfaces of 
the higher vertebrates such as those of the mouth and 
the nose. Moreover when these endings are rendered 
inoperative by cutting their nerve trunks, as Sheldon did 
on the dogfish and as has often been done on the nasal 
cavities of mammals, irritating substances are no longer 
effective. Free-nerve endings of spinal or cranial nerves 
are, therefore, quite certainly the type of nerve-terminal 
concerned with the reception of chemical irritants. 

4. Relation to Other Senses. In discussing the relation 
of the receptors for chemical irritants to other sense 
organs some of the earlier workers suggested a compari- 



106 SMELL, TASTE, ALLIED SENSES 

son of these receptors with those for taste Parker 
(1908a); Herrick (1908). More recently Cogliill (1914) 
has declared that since tactile and chemical irritability 
develop simultaneously in certain amphibian larvae, 
chemical irritability is in reality tactile in nature. It 
must also be perfectly evident that the receptors under 
consideration have striking resemblances to those con- 
cerned with pain. 

The fact that organs of taste always involve special- 
ized end-organs, such as taste-buds, whereas receptive 
surfaces for chemical irritants may contain only free- 
nerve endings, shows that the relation of these two 
classes of receptors is at best only distant. This con- 
clusion is supported by an observation by Parker and 
Stabler (1913) that the minimum concentration of ethyl 
alcohol necessary for the stimulation of the irritant 
receptors in man, 5 to 10 molar, is decidedly stronger 
than that which will stimulate the human gustatory 
organs, 3 molar. 

The relation of the receptors for irritants to those 
for touch and for pain seems to be clearly indicated in 
the results of experiments in which exhaustion and nar- 
cotics have been used. If the tail of an amphioxus is 
subjected to about twenty applications of weak nitric 
acid, 0.025 molar, in fairly rapid succession, the fish will 
cease to respond to this kind of stimulus. After the 
exhaustion of the mechanism for this type of reception, 
the tail of the fish will be found fully sensitive to the 
touch of a camel's hair brush. If, now, the tail of 
a fresh individual is vigorously stroked some thirty times 
in succession, the fish will cense to respond to this form 
of mechanical stimulation but it will still be found very 



THE COMMON CHEMICAL SENSE 107 

sensitive in the exhausted part of the skin to weak acid. 
Thus mechanical stimulation and chemical stimulation 
seem to apply to different sets of terminals and the 
exhaustion of one set does not involve that of the other. 

On treating a portion of the surface of a dogfish with 
2 per cent cocaine, Sheldon found that tactile stimulation 
ceased in from ten to twenty minutes whereas chemical 
stimulation was effective for a somewhat longer period. 
By continuing the treatment with cocaine receptivity for 
chemical irritants was also eventually abolished. In a 
similar way Cole (1910) found that if the hind foot of a 
spinal frog was treated with 1 per cent cocaine till the 
animal no longer responded to pricking or scratching 
with a needle or to pinching with forceps, it would never- 
theless respond vigorously to a salt solution. The'se 
results were confirmed by Crozier in 1916 who used a 
half per cent solution of cocaine hydrochloride on a 
frog's foot. After about 20 minutes' immersion in this 
solution, the reaction time of the cocained foot to formic 
acid 0.05 molar, was about twice that of the normal foot. 
After about an hour to an hour and a half of this treat- 
ment the cocained foot no longer reacted to pinching but 
gave good responses to acid with reaction times of from 
ten to fifteen seconds, about twice that of the non-cocained 
foot. These observations show beyond a doubt that the 
effect of chemical irritants on the naturally moist skin 
of vertebrates is not to be ascribed to the stimulation 
of organs of touch or of pain but to some other form of 
receptor, the terminals of what has been called the com- 
mon chemical sense. 

As Crozier has pointed out, there can be no question 
of the distinctness of the human sensations attributed 



108 SMELL, TASTE, ALLIED SENSES 

to the common chemical sense as contrasted with our 
sensations of smell, taste, touch, or pain. The curious 
feeling that comes from vapors that irritate the eyes, 
nose, or even the mouth has not the remotest relation 
to touch, smell, or taste and is only distantly suggestive 
of pain. Pain, however, is easily separated from the 
common chemical sense by the use of cocaine, and we 
are, therefore, entirely justified in concluding that the 
common chemical sense is a true sense with an indepen- 
dent set of receptors and a sensation quality entirely its 
own. In the fishes and amphibians it pervades the whole 
integument but in the reptiles, birds and mammals it is 
restricted to the partly exposed mucous membranes of the 
natural apertures, a restriction that doubtless arose as 
the vertebrate changed from an aquatic to an air-inhabit- 
ing form. 

5. BIBLIOGRAPHY 

BBAEUNIN T G, H. 1904. Zur Kennitnisa der Wirkung chemischer Reize. 
Arch. ges. Physiol., Bd. 102, pp. 163-184. 

COGHILL, G. E. 1914. Correlated Anatomical and Physiological Studies 
of the Growth of the Nervous System of Amphibia, I. The Afferent 
System of the trunk of Amblystoma. Jour. Comp. Neurol., vol. 24, 
pp. 161-233. 1919. II. The Afferent System of the head of Ambly- 
stoma. Jour. Comp. Neurol., vol. 26, pp. 247-340. 

COLE, L. \V. 1910. Reactions of Frogs to Chlorides of Ammonium, Potas- 
sium, Sodium, and Lithium. Jour. Comp. Neurol. Psychol., vol. 20, 
pp. 601-614. 

CROZIEB, W. J. 1915. Ionic Antagonism in sensory Stimulation. Amer. 
Jour. Physiol, vol. 39, pp. 297-302. 

CROZIKR, W. J. 1916. Regarding* the Existence of the " Common Chemical 
Sense " in Vertebrates. Jmir. Comp. Neurol., vol. 26, pp. 1-8. 

HERRICK, C. J. 1908. On the phylogenetic Differentiation of the Organs 
of Smell and Taste. Jour. Comp. Neurol. Psycho*., vol. 18, pp. 159-166. 

LOEB, J. 1905. On the Production and Suppression of Muscular Twitch- 
ings arid Hypersensitiveness of the skin by Electrolytes. Studies in 
<:neral Physiology, vol. 2, pp. 748-765. 



THE COMMON CHEMICAL SENSE 109 

NAGEL, W. 1894. Vergleichend physiologische und anatomische Unter- 
suchungen iiber den Geruchs- und Geschmackssinn und ihre Organe. 
Bibl. Zool., Heft 18. 

PARKER, G. H. 1908a. The Sense of Taste in Fishes. Science, vol. 27, 
p. 453. 

PARKER, G. H. 1908b. The Sensory Reactions of Amphioxus. Proc. 
Amer. Acad. Arts. Sci., vol. 53, pp. 415-455. 

PARKER, G. H. 1912. The Relation of Smell, Taste, and the Common 
Chemical Sense in Vertebrates. Jour. Acad. Nat. Sci. Philadelphia, 
vol. 15, pp. 221-234. 

PARKER, G. H. and E. M. STABLER. 1913. On Certain Distinctions be- 
tween Taste and Smell. Amer. Jour. Physiol., vol. 32, pp. 230-240. 

SHELDON, R. E. 1909. The Reactions of the Dogfish to Chemical Stimuli. 
Jour. Comp. Neurol. Physchol., vol. 19, pp. 273-311. 



CHAPTEE VI. 

ANATOMY OF THE GUSTATORY ORGAN. 

Contents. 1. Distribution of Taste-buds in the Oral 
Cavity of Man. 2. Comparative Distribution of Taste- 
buds. 3. General Form of Taste-buds. 4. Cellular Com- 
position of Taste-buds. 5. Intragemmal and Other 
Spaces. 6. Innervation of Taste-buds. 7. Gustatory 
Nerves. 8. Eelation of Gustatory Nerve Fibers and 
Taste-buds. 9. Bibliography. 

1. DISTRIBUTION of Taste-buds in the Oral Cavity of 
Man. In man the organs of taste are located in the 
mouth. These are the so-called taste-buds discovered 
independently by Loven (1867) and by Schwalbe (1867). 
In the adult human being they have been identified on 
the dorsal surface of the tongue except the mid-dorsal 
region, on both the anterior and posterior surfaces of the 
epiglottis, on the inner surface of the arytenoid process 
of the larynx, on the soft palate above the uvula, on the 
anterior pillars of the fauces, and on the posterior wall 
of the pharynx. All other oral surface in the adult, such 
as the lips, the gums, the cheeks, the inferior surface of 
the tongue, the hard palate, the uvula, and the tonsils 
are devoid of these organs. 

In young individuals, babes, and human embryos 
taste-buds are more widely distributed than they are in 
the adult. According to Tuckerman (1890a, 1890b) and 
Graberg (1898) taste-buds appear in man at about the 
beginning of the third month of foetal life. Stahr (1902) 
found them in human embryos in the middle of the dor- 
no 



ANATOMY OF THE GUSTATORY ORGAN 111 

sum of the tongue and Ponzo (1905) identified them on 
the palatine tonsils, the hard palate, and the cervical 
part of the esophagus, regions from which they are 
absent in the adult. As early as 1875 Hoffmann called 
attention to the fact that in human embryos and newly 
born babes taste-buds were commonly found on the free 
surfaces of the vallate papillae, situations from which 
they disappear in later life. This observation was con- 
firmed by Tuckerman (1889) as well as by Hermann 
(1885), who, however, worked upon the rabbit. Thus the 
gustatory apparatus of man and of other mammals is 
by no means constant, but suffers reduction from the late 
embryonic period to the adult state. On the tongue of 
man the reduction is chiefly in the middle region of the 
distal two-thirds so that, as Stahr (1902) has pointed 
out, the center of taste in this organ shifts with growth 
from a position near the tip of the tongue to one in the 
neighborhood of the vallate papillae. This opinion is in 
agreement with the observation of Heiderich (1906) that 
after birth the taste-buds of the vallate papillae show 
almost no change. 

Wherever taste-buds occur in man, except on the 
tongue, they are found simply imbedded in the epithe- 
lium of the mucous membrane of the region concerned. 
On the tongue, however, they are almost invariably asso- 
ciated with certain kinds of papillae. The human tongue 
possesses several classes of these structures, which from 
their forms have been designated as conical, filiform, 
fungiform, foliate, and vallate. The plush surface of the 
dorsum of the tongue is produced by innumerable fine 
conical and filiform papillae. These, however, almost 
never have taste-buds associated with them. The other 



112 



SMELL, TASTE, ALLIED SENSES 



types of papillae, the fungiform, foliate, and vallate, very 
generally carry taste-buds (Fig. 25). 

The fungiform papillae are relatively large knob-like 
elevations scattered over the dorsum of the tongue. 
They can be easily seen with the unaided eye and may 
be readily located and identified. They commonly carry 

a few taste-buds embed- 
ded in the epithelium of 
their free outer sur- 
faces. In sections of the 
crowns of these papillae 
parallel to the surface 
of the tongue three or 
four or more, rarely six 
to eight, taste-buds may 
be identified. In verti- 
cal section it can be 
seen that the taste-buds 
are not indiscriminately 
scattered over the free 
surface of the papilla, 
but are perched on the 
secondary dermal pa- 
pillae contained within the papilla proper and that they 
always reach through the full thickness of the epidermis 
from the dermal core of the secondary papilla to the free 
outer surface of the primary papilla itself (Fig. 26). 
This extension through the whole thickness of the epi- 
dermis seems to be a common characteristic of taste- 
buds, for it is to be noted in them from fishes to man. 
It is an easy means of distinguishing them from other 
bud-like receptors such as the lateral-line organs whose 




Fio. 25. Dorsal view of the human tongue 
ehowing foliate papillw (f) and vallate papilla (v). 



ANATOMY OF THE GUSTATORY ORGAN 113 

cells extend only part way through the epithelium in 
which they are imbedded. 

The foliate papillae lie on either side of the edge of 
the human tongue and close to its root. They form a 
series of from three to eight vertical parallel ridges. 
Each ridge is abundantly supplied with taste-buds which, 








Fio. 26. Vertical section of a fungiform papilla showing two taste-buda. 

however, do not occur on its free outer surface but on 
its sides. Here the buds open into the ditch between the 
ridge on which they are located and the next one. In 
sections transverse to the axis of the ridge the numbers 
of taste-buds seen on the two sides of a given ditch may 
vary from three to twenty. In the rabbit the foliate pa- 
pillae are especially well developed and are abundantly 
supplied with taste-buds. These have been very fully 
studied recently by Heidenhain (1914) who has shown 
that the buds are arranged in more or less vertical rows 
on each papillar fold and that they probably increase 
in numbers by a process of fission. 

The vallate papillae, which in man are usually six to 
twelve in number, form on the posterior part of the 

8 



114 SMELL, TASTE, ALLIED SENSES 

tongue a V-shaped row whose angle points toward the 
esophagus (See Fig. 25). Each papilla is a low circu- 
lar elevation surrounded by a relatively deep, narrow 
ditch. The taste-buds are located on the walls of this 
ditch and chiefly on that wall which forms part of the 
papilla. In a vertical section through a vallate papilla, 
it is usual to see on the side of the ditch formed by the 




FIG. 27. Vertical section of a vallate papilla showing taste-buds. 

papilla from ten to a dozen taste-buds and on the side 
away from that structure four to six such bodies 
(Pig. 27). However, as Schwalbe (1868) long ago 
pointed out, much individual variation occurs and it is, 
therefore, very difficult even to estimate with any degree 
of accuracy the total number of taste-buds on a single 
papilla. W. Krause (1876) believed the number for a 
single papilla in man to be as high as 2500, but von Wyss 
(1870) placed it much lower, namely, at about 400. Even 
these figures seemed too high to Graberg (1899) who gave 
the maximum at 100 to 150 and the minimum at 40 to 50. 
Heiderich (1906) made a close count on 92 papillae .from 
human beings ranging in age from the first to the twen- 
tieth year and found the extreme numbers of buds to a 



ANATOMY OF THE GUSTATORY ORGAN 115 

papilla to be 508 and 33 with an average not far 
from 250. 

2. Comparative Distribution -of Taste-buds. Taste- 
buds, like the olfactory receptors, require a moist sur- 
face. It is, therefore, not surprising to find that in all 
air-inhabiting vertebrates they are limited to the oral 
cavity. Their distribution in mammals has been very 
fully studied by Tuckerman (1892), Munch (1896), and 
Haller (1909). 

Taste-buds also appear to be limited to the oral 
region in amphibians notwithstanding the fact that many 
of these animals possess a permanently moist skin. In 
fishes they were apparently first seen by Leydig in 1851 
and were subsequently described by Schulze (1863). In 
these forms they are not restricted to the oral region. 
According to Johnston (1906) they are present on the 
heads of cyclostomes as well as on those of ganoids where 
they were studied by Dogiel (1897). Herrick (1918) 
states that in some bony fishes, such as the catfishes, the 
carps, and the suckers they are to be found over the 
entire outer surface of the body and this investigator 
(1903) has further shown that in the catfish Amiurus the 
taste-buds on the flank of the fish are as significant in the 
detection of bait as are those about the mouth (Fig. 28). 

3. General Form of Taste-buds. Taste-buds vary in 
form from that of a flask to that of a. spindle. Commonly 
they are single bud-shaped bodies opening to the exte- 
rior by a small pore (Fig. 29). Compound buds in which 
the body of the bud appears double and two pores are 
present have long been known and Heidenhain (1914) has 
recently shown that this condition may reach an extreme 
degree of complexity in the foliate papillae of the rabbit 



116 SMELL, TASTE, ALLIED SENSES 

where compound buds with as many as six pores have 
been identified. The frequency with which types of buds 
with different numbers of pores occur may be gathered 
from the enumeration by Heidenhain who found that in 
509 taste-buds from the foliate papillfe of the rabbit 
368 had one pore, 100 two pores, 29 three pores, 7 four 




Fio. 28. Lateral view of the catfish, Amiurus melas, showing in black the gustatory 
branches of the facial nerve. After Herrick, 1903, Fig. 3. 

pores, 1 five pores, and 4 six pores. In the compound 
buds the pores usually form a more or less linear series 
and as each pore represents a single element in the com- 
plex the whole gives the impression of a row of fused 
buds (Fig. 30.) These compound buds are believed to 
result from a process of imperfect division. 

Some taste-buds open directly on the oral surface 
where they are located ; others are marked by a pore, the 
outer taste-pore, which leads into a short canal and this 
in turn ends at the inner taste-pore formed by the distal 
end of the bud itself. Von Ebner (1897) noted that in 
some instances the canal expanded into a small chamber 
or ampulla over the tip of the bud and, though Grabcrg 
(1899) could not. confirm this statement for man, the 
condition has been observed anew by Kallius (1905) in 



ANATOMY OF THE GUSTATORY ORGAN 117 




human material and by Heidenhain (1914) in the rabbit. 

5. Cellular Composition of Taste-buds. The cells 
composing the taste-buds are so arranged as to give each 
bud somewhat the appearance of a flower bud or of a 
leaf bud not yet unfolded. As has been stated already, 
these end-organs were described in the skin of fishes as 
early as 1851 by Leydig and were 
subsequently simultaneously and 
independently discovered in the 
mouths of the higher vertebrates 
<by Loven (1867) and by Schwalbe 
(1867). The older workers 
usually distinguished in the taste- 
buds two classes of cells, taste- 
cells, which were supposed to be 
chiefly central in position, and 
supporting cells mainly on the 
exterior of the bud. 

Each taste-cell is an attenuated delicate structure 
whose elongate nucleus forms a slight enlargement near 
the middle of the cell-body (See Fig. 31a). Distal to 
it narrows to a delicate process, the taste hair. This 
hair either projects out of the pore into the exterior or 
into the canal when that is present. Proximal to the 
nucleus the taste-cell extends into the deeper part of the 
bud there to terminate usually in a small rounded knob. 
The number of taste-cells in a bud varies from two or 
three to as many as the contained supporting cells, per- 
haps ten or more. 

Beside the taste-cells proper Schwalbe (1867) de- 
scribed what he believed to be a second form of receptive 



Fio. 29. A simple taste-bud 
from a foliate papilla of the 
rabbit. After Heidenhain, 1914, 
Plate 19, Fig. 5. 



118 



SMELL, TASTE, ALLIED SENSES 




Fio. 30. A compound] taste- 
bud from a foliate papilla of the 
rabbit. After Heidenhain 1914, Plate 
23, Fig. 27. 



cell to which he gave the name of "Stabzelle" or rod cell. 
This type of cell was said to differ from the ordinary 
taste cell in that it was without a taste hair. It has not been 
identified with certainty by subsequent investigators. 

The supporting cells of the taste-buds have been the 
occasion of much difference of opinion. The older 

workers believed that these 
cells were limited to the exte- 
rior of the buds, but Merkel 
(1880) showed that they also 
occurred in the interior and 
Eanvier (1888) . definitely 
described both inner and outer 
supporting cells. Hermann 
(1889) concluded that these 
two classes of supporting cells 
differed not only in position but also in structure. 
The outer cells, which he called pier cells (Pfeilerzellen), 
were relatively large pyramidal elements whose 
nuclei were proximal in location and whose distal 
ends terminated in a zone marked with fine vertical 
stripings. For the inner supporting cells Hermann used 
Schwalbe's term of rod cells (Stabzellen) without, how- 
ever, wishing thereby to imply that they were of a sensory 
nature. They were said to differ from the pier cells 
in that they were devoid of the peripheral striped zone. 
Hermann also described basal supporting cells which to 
the number of two to four were found in the proximal 
part of the taste-buds. Von Lenhossek (1893b) doubted 
the existence of basal cells and described four not very 
sharply separate types of supporting cells. Graberg 
(1899) reidentified in human material the basal cells dis- 



ANATOMY OF THE GUSTATORY ORGAN 119 

covered by Hermann. The other supporting cells were 
described by this author as either central or peripheral 
and were to be distinguished from each other rather by 
location than by differences of structure. 

The indefiniteness and uncertainty that surrounded 
the question of the classes of supporting cells in taste- 
buds has been dissipated in large part by the declaration 
of Kolmer (1910) that between the taste-cells on one 
hand and the so-called supporting cells on the other there 
are all possible transitions and that it is, therefore, a 
mistake to attempt to draw distinctions not only between 
various kinds of supporting cells but between supporting 
cells and taste-cells. Kolmer believed that all the elon- 
gated cells in taste-buds are really taste-cells and that 
their differences are due to the stage of growth at which 
they are for the moment. This opinion, which is sup- 
ported by what is known of the innervation of the taste- 
buds, has gained the acceptance of the more important 
recent workers in this field, such as Retzius (1912) and 
Heidenhain (1914). If true, it shows the taste-bud to be 
a much more unified structure than has heretofore been 
supposed and it does away at once with the confusion 
over the classes of cells that were believed to enter into 
its composition. 

The basal cells apparently do not fall into this general 
category of more or less differentiated receptor cells, 
but, according to Heidenhain at least, they are elements 
that only under certain conditions are regularly present 
and are concerned with the division of the buds. 

The epidermal cells immediately next the taste-bud 
are often flattened against this structure and conform 
more or less to its outline. These cells have been called 



120 SMELL, TASTE, ALLIED SENSES 

o 

by Grabcrg (1899) extrabulbar cells and though they are 
not to be classed as part of the bulb proper they are 
nevertheless sufficiently related to that structure to be 
appropriately mentioned in this connection. 

As Hermann (1889) long ago pointed out, the cells of 
the taste-buds are probably undergoing continual change. 
Old cells are degenerating and disappearing and new 
ones are forming to take the places of those that have 
broken down. The degenerating process is indicated by 
the presence in the taste-buds of cells in all stages of 
growth and of considerable numbers of leucocytes, as 
pointed out by Ranvier (1888), von Lenhossek (1893b), 
and others. The regenerative process is shown in the 
occasional occurrence of mitotic figures in the base of 
the bud thus giving evidence of cell division in that region 
Hermann (1889). 

5. Intragemmal and other Spaces. Graberg (1899) 
has called attention to the fact that taste-buds are not 
solid structures but that their cells are separated one from 
another by considerable intervening space, and that 
much free space occurs in the tissue immediately around 
the buds. This intra-, peri-, and subgemmal space is be- 
lieved by Graberg not to be an artifact, for it can be 
identified by almost all methods of preparation. Accord- 
ing to this investigator these various spaces communicate 
with one another and connect with the exterior through 
the taste pore. They may be the means of irrigating and 
thereby cleaning the taste-bud, for it is possible that 
fluid may flow slowly through them from the interior to 
the exterior. 

6. Innervation of Taste-buds. Among the older in- 
vestigators the innervation of the taste-buds was a ques- 



ANATOMY OF THE GUSTATORY ORGAN 121 

tion of much uncertainty. Some claimed that the 
gustatory nerve-fibers connected directly with the cells 
of the taste-buds; others that they did not so connect. 
The first to employ special neurological methods for the 
solution of this question were Fusari and Panasci (1890). 
These workers claimed that by means of Golgi prep- 





FIG. 31. Golgi preparations of the taste-buds of the rabbit, a showing cells (after 
von Lenhosse'k, 1893a, Fig. la) and 6 showing nerve-terminations (after Retzius, 1892a, 
PlateS, Fig. 4). 

arations it could be shown that the gustatory cells were 
directly connected with nerve-fibers. Two years later 
Retzius (1892a) published an account of the innervation of 
the taste-buds of mammals and of amphibians in which 
he showed in preparations stained by methylenblue as 
well as by the Golgi process that the nerve-fibers were 
not directly connected with the taste-cells but ended in 
close proximity to them (Fig. 31). These results were 
confirmed in 189 3 by von Lenhossek, Arnstein, and Jacques 
as well as by the subsequent work of Retzius himself 
(1893) and there seems to be no ground for doubting 
the correctness of the general conclusion arrived at more 
or less independently by these four investigators. 

The anatomical relations shown by these workers 
are relatively simple. From the subepithelial nerve 
plexus in the neighborhood of taste-buds fibers pass out- 
ward into the epidermis. These fibers either form sys- 



122 



SMELL, TASTE, ALLIED SENSES 



terns of branches ending in free terminations around a 
taste-bud, in which case they are called perigemmal or 





Fio. 32. Golgi preparations of the taste-buds of the cat, a, in longitudinal section 
fihowing nerve terminations, and 6, in transverse section showing intrageinmal nerve 
ebers. After Retzius, 1892a, Plate 7, Figs. 1 and 4. 

peribulbar fibers, or they enter the bud and end freely 
among its cells being designated then as intragemmal 
or intrabulbar fibers (Fig. 32). As the figures given by 
Retzius, Arnstein, and others show, the nerve-fibers in 



ANATOMY OF THE GUSTATORY ORGAN 123 

the buds are as intimately applied to the so-called sup- 
porting cells as to the taste-cells, showing, as has al- 
ready been stated, that the distinction between what 
has been assumed to be two classes of cells is probably 
quite erroneous. 

In addition to intergemmal and iperigemmal fibers, 
which in consequence of their close relations with the 
taste-buds may be designated as gemmal or bulbar fibers, 
there are also fibers that pass into the undifferentiated 
epithelium between the buds and end close to the external 
surface as free-nerve terminations. These have been 
called intergemmal fibers, but it is doubtful whether 
they have anything to do with taste and it is not improb- 
able that they are concerned with other sensory functions 
such as the common chemical sense, pain, and the like, in 
which case a designation implying relations to a taste-bud 
is in no sense appropriate. 

Taste-buds such as have already been described have 
been found in a wide range of vertebrates. They 
not only occur in mammals, where their relation with 
the nerve-fibers was first correctly described by Retzius 
( 1892a) , but also in fishes as seen by Retzius ( 1892a, 1893 ) , 
vonLenhossek (1893a),Dogiel (1897) and others (Fig. 33). 
It is, therefore, probable that so far as essentials are con- 
cerned the innervation of the taste-buds of all vertebrates 
presents a relatively uniform plan. 

7. Gustatory Nerves. There are no separate gusta- 
tory nerves in the vertebrates as there are olfactory nerves 
or optic nerves. Gustatory fibers occur in several crani- 
al nerves and it is by means of these that the taste-buds 
of various regions are provided with those nervous con- 
nections that have been described in the preceding section. 



SMELL, TASTE, ALLIED SENSES 




In the fishes the nerves chiefly concerned are the vagus, 
the glossopharyngeal and the facial. The taste-buds of 
the gill region are supplied by the vagus and the glosso- 
pharyngeal. Those that are in the mouth proper or are 
on the exterior of the body are innervated by the facial 
ni 1-ve. Consequently in the catfish (See Fig. 28), in 

which the whole outer 
skin is provided with 
taste-buds, this nerve is 
enormously developed and 
sends large branches to 
the barbels and an exten- 
sive recurrent branch to 
the flanks of the body 
(Herrick, 1903). 

In mammals, includ- 

. 

Hlg mail, tllC llinerVatlOll 

of the taste-buds is not 
upon so simple a plan as in fishes. In these higher 
vertebrates gustatory fibers may possibly be contained 
in four of the cranial nerves, the vagus, the glosso- 
pharyngeal, the facial, and the trigeminal. The 
distribution of these nerves in the human tongue has been 
worked out by Zander (1897). Certain parts of the 
vagus are distributed to the larynx and to the epiglottis 
as well as to the most posterior part of the tongue itself 
and innervate very probably the taste-buds of these re- 
gions (Fig. 34). The glossophaiyngeal supplies the pos- 
terior third of the tongue including the foliate and vallate 
papillae, for, as was first shown by von Vintschgau and 
Honigsf.hmied (1876), when the ninth nerve is cut the 
taste-buds of these parts soon degenerate and disappear. 
Although the correctness of this observation was denied 



Fio. 33. Golgi preparations of the taste- 
buds of the common European barbel show- 
ing cells and nerve-fibers. After von Lenhossek, 

1893 a . Fig. 2. 



ANATOMY OF THE GUSTATORY ORGAN 



125 



by Baginsky (1894), it has been confirmed by such a 
number of observers, including Drasch (1887), Ranvier 
(1888), Sandmeyer (1895), Meyer (1897) and others, that 
it is now generally accepted. Both the right and the 
left branches of this nerve innervate the median vallate 
papilla in mammals and form at the base of this organ, 
as Vastarini-Cresi (1915) has 
shown, more or less of a gusta- 
tory chiasma. The anterior two- 
thirds of the tongue in man are 
innervated by the lingual nerve 
which is made up of a union of 
the lingual branch of the trigem- 
inal nerve with the chorda 
tympani of the facial. It has 
been an open question whether 
the gustatory fibers for this part 
of the tongue belong to the 
trigeminal, to the facial, or 
possibly even to the glosso- 
pharyngeal, for all these nerves 
intercommunicate through a 
plexus of fine branches near their roots. F. Krause (1895) 
noted the effect on taste of the complete extirpation of the 
ganglion of the trigeminal nerve, and found that in some 
instances taste was entirely obliterated from the appro- 
priate part of the tongue, but that in others it was only 
somewhat reduced. These differences do not appear in 
the later and more conclusive work of Gushing (1903) 
who found that, when time enough was given, all subjects 
from whom the ganglion of the trigeminal nerve had 
been removed, recovered taste completely. He attributed 




FIG. 34. Diagram of the 
human tongue showing the parts 
innervated by the lingual nerve 
(horizontal lines), by the glosso 
pharyngeal nerve (oblique lines), 
and by the vagus nerve (small 
circles). After Zander, 1897. 



126 



SMELL, TASTE, ALLIED SENSES 



the temporary disturbance in taste, a condition that was 
supposed to be permanent by Krause, to the effect of the 
degenerating trigeminal fibers on the adjacent gusta- 
tory fibers, an effect that disappeared when the degenera- 
tion was complete. Consequently Gushing concluded that 
the gustatory fibers from the anterior part of the tongue 



VII 




FIG. 35. Diagram to illustrate the possible paths of the gustatory nerve-fibers from the 
tongue to the brain in man. The distal part of the tongue (1) is innervated by the lingual 
nerve (2) whose gustatory fibers pass to the brain by way of the chorda tympani (3), a 
branch of the facial nerve (VII). The proximal part of the tongue is innervated^ by the 
glossopharyngeal nerve (4). The undoubted gustatory paths over the facial nerve (VII) and 
the glossopharyngeal nerve (IX) are indicated by dotted lines. The commonly assumed 

Eaths by way of the trigeminal nerve (V) are shown in heavy black lines with arrows. Modi- 
ed from Cushing, 1903. 

are not part of the trigeminal nerve. If this is so, they 
must belong to the facial or possibly to the glossopharyn- 
geal nerve (Fig. 35). That they are abundantly present 
in the chorda tympani of the facial nerve is well known 
from the fact that direct stimulation of the chorda in 
the neighborhood of the ear drum is commonly accom- 
panied by sensations of taste, but whether these gusta- 
tory fibers on reaching the facial nerve pass into the brain 
through its root or make their way to the root of the glos- 
sopharyngeal is not yet definitely settled. It is, therefore, 
probable that in mammals the trigeminal nerve, though 



ANATOMY OF THE GUSTATORY ORGAN 127 

suspected of including gustatory fibers, is really devoid 
of them. These fibers at most occur in the facial, glos- 
sopharyngeal and vagus nerves, but none of these nerves 
is exclusively gustatory. 

8. Eelation of Gustatory Nerve-fibers to Taste-buds. 
It is an interesting and significant fact that on the de- 
generation of the gustatory nerve-fibers the taste-buds 
associated with them should disappear. This state of 
affairs, long ago demonstrated for mammals, has recently 
been shown by Olmsted (1920a, 1920b) to occur also in 
fishes. Meyer (1897) showed that thirty hours after cut- 
ting the glossopharyngeal nerve in the rabbit the taste- 
buds began to show a change and that by the end of seven 
days most of them had disappeared. In the catfish Ami- 
urus, according to Olmsted, the taste-buds on the oral 
barbels begin to degenerate in a little over ten days after 
the nerve to these organs has been cut and they com- 
pletely disappear by the end of the thirteenth day. 
Ranvier (1888) believed that in mammals the taste-buds 
were destroyed by wandering cells, but Sandmeyer (1895) 
and Meyer (1897) held the view that the gustatory cells 
suffered dedifferentiation and changed into ordinary epi- 
thelial cells. In Amiurus Olmsted has found strong 
evidence in favor of the destruction of the cells of the 
taste-buds by phagocytes thus supporting Ranvier 's 
original opinion. 

Olmsted has shown, further, that on the regeneration 
of a nerve in a denervated Amiurus barbel from which 
all the taste-buds had disappeared, new buds reappear 
coincident with the arrival of the nerve. With the de- 
generation of the nerve and the loss of the taste-buds the 
barbels lose their receptivity for sapid materials, nor 



128 SMELL, TASTE, ALLIED SENSES 

does this return till the buds regenerate. Since the taste- 
buds disintegrate with the loss of the nerve and new ones 
form only with the regeneration of this structure, it is 
clear that the bud is dependent upon the nerve. As 
Olmsted has suggested, it is probable that when a twig of 
the nerve reaches a given spot in the epidermis, it gives 
out a substance, hormone-like in character, that excites 
the epithelial cells of that spot to form a bud much as the 
embryonic eye cup of the vertebrate excites in the super- 
imposed ectoderm the formation of a lens. In this way at 
least the intimate dependence of the taste-bud on the re- 
generating nerve-fiber can be explained and,, judging 
from the account given by Landacre (1907) of the ontoge- 
ny of these organs, a similar explanation may also 
apply in development. 

9. BIBLIOGRAPHY. 

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O 

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CHAPTER VII. 

PHYSIOLOGY OF GUSTATION. 

Contents. 1. Location of Taste. 2. Gustatory stimulus. 
3. Qualities of Taste. 4. The Sour Taste. 5. The Saline 
Taste. 6. The Bitter Taste. 7. The Sweet Taste. 8. 
Inadequate Stimuli. 9. Distribution of Tastes on the 
Tongue. 10. Action of Drugs on Taste. 11. Substances 
with two Tastes. 12. Latency of Taste Sensations. 13. 
Taste Alterations ; After-tastes. 14. Gustatory Contrasts. 
15. Taste Compensations and Mixtures. 16. The Gusta- 
tory Senses. 17. Comparative. 18. Bibliography. 

1. LOCATION of Taste. Although in man taste is not 
strictly limited to the mouth, for it spreads into some 
of the adjacent cavities, it is primarily located in the 
buccal space and is especially a function of the tongue. 
When the mouth of a normal adult is explored witli solu- 
tions of sapid substances, many parts such as the lips, 
the gums, the floor, the lower surface of the tongue, the 
inner surfaces of the cheeks, and the hard palate are 
found to be insensitive to taste. Even the uvula which, 
according to many of the older workers, was regarded as 
having a gustatory function, has been shown by Kiesow 
and Hahn (1901) not to be concerned with taste. All 
these regions are well known to be devoid of taste-buds. 
Whether the pillars of the fauces and the tonsils have 
to do with taste is a matter of dispute. Hanig (1901) 
believed that these parts have a gustatory function, but 
Kiesow and Hahn (1901) regarded them as usually in- 
sensitive. The mucous membranes of the following parts 

132 



PHYSIOLOGY OF GUSTATION 133 

are concerned with taste; the beginning of the gullet, 
the region of the arytenoid cartilages within the larynx, 
the epiglottis, the soft palate, and particularly the tongue. 
In all these regions taste-buds have been identified. On 
the tongue of adult human beings taste is limited to the 
tip, the lateral margins and the dorsal surface of the 
root, the large central area on the upper surface of this 
organ being devoid of taste. In children, as contrasted 
with adults, the whole upper surface of the tongue in- 
cluding the central area is said to be sensitive to taste 
as is also the inner surfaces of the cheeks. 

2. Gustatory stimuli. The stimulus for taste is an 
aqueous solution of a great variety of substances. Mate- 
rials insoluble in water are tasteless,, but not all substances 
that form aqueous solutions have taste. Thus oxygen, 
hydrogen, and nitrogen, though freely soluble in water, 
are without taste. Piutti (1886) long ago showed that 
Isevo-asparagine is tasteless, although its stereoisomer 
dextro-asparagine is sweet. Other organic compounds, 
such as the carbohydrates raffinose and alpha-galaoctite 
are said to be almost, if not quite, tasteless. 

When solids or semi-solids are chewed in the mouth, 
they not only become mixed with the saliva whereby many 
of their components become dissolved, but they are 
spread over the surface of the tongue and are thus 
brought into intimate contact with its taste-buds. In fact 
it is not improbable that the movement of the tongue fa- 
cilitates the entrance of these solutions into the pores of 
the taste-buds. At least solutions placed upon the 
tongue, particularly near its root, are tasted with greater 
certainty, when this organ is moved about than when it 
is held still. 



134 SMELL, TASTE, ALLIED SENSES 

3. Qualities of Taste. Tastes, unlike odors, fall into 
a limited number of well-circumscribed groups, which 
have received distinctive names such as sour, saline, bit- 
ter, sweet, and the like. The multitude of flavors and 
other sensations associated with our food are undoubt- 
edly mixed in character and include touch, heat, cold, the 
common chemical sensation, and especially odor. By ap- 
plying materials in weak solution, at the temperature of 
the mouth and with the nostrils closed, extraneous sen- 
sations may be eliminated and there remains a certain 
irreducible residue, the tastes. Zenneck (1839), Valentin 
(1848), Duval (1872) and later Sternberg (1898) admit- 
ted only two classes of tastes, sweet and bitter. Stich 
(1857), however, long ago showed that sour was a sensa- 
tion produced by stimulating only a limited part of the 
buccal surface, and Schiff (1867) made the important 
observation that a solution of acid too weak to stimulate 
the general mucous surface would nevertheless call forth 
a sour sensation when it was applied to the gustatory 
region. Von Vintschgau (1880) made similar observa- 
tions concerning the saline taste; solutions of sodium 
chloride, potassium iodide, and ammonium chloride, if 
sufficiently weak, will stimulate the organs of taste, but 
if strong they will stimulate not only these organs but 
the nerve endings of the general buccal cavity also. In 
consequence of such observations sour and saline are now 
universally included with bitter and sweet as true tastes. 

In addition to these four tastes there are a number 
of questionable ones such as metallic and alkaline, tastes 
that were originally accepted by Wundt (1887) among 
others. The so-called metallic taste is excited by solu- 
tions of salts of the heavy metals, silver, mercury, and 



PHYSIOLOGY OF GUSTATION 135 

the like (Kahlenberg, 1898). The metallic taste of a 
0.0005 molar solution of silver nitrate is very pronounced 
and is discernible even at the greater dilution, 0.0002. 
Since the nitrate ions are incapable of exciting taste at 
such slight concentrations, it follows that stimulation 
must depend upon the silver ions. In a similar way mer- 
cury ions in normal solutions of 0.001 to 0.0005 of mercu- 
ric chloride have been shown to excite the so-called 
metallic taste. This taste, however, has been declared 
to be a complex of other tastes such as sour and sweet, 
and Herlitzka (1808) has gone so far as to maintain that 
it is not a true taste but an olfactory phenomenon. 

The alkaline tastes so-called are excited by the appli- 
cation to the tongue of dilute solutions of such caustic 
alkalis as sodium or potassium hydrate. Kahlenberg 
(1898) has shown that the stimulating material in such 
mixtures is the hydroxylion which is effective in solutions 
as weak as 0.0025 molar. In the alkaline taste, as in the 
metallic taste, the results have been variously explained. 
Oehrwall (1891) regarded the so-called alkaline taste as 
a mixture of sensations due to a simultaneous combina- 
tion of several tastes and touch. Hober and Kiesow 
(1898) pointed out that weak alkalis produce a sweetish 
taste, but von Frey (1910) showed that these reagents 
act on the tongue in such a way as to produce odorous 
materials that he believed to be the occasion of the so- 
called alkaline taste. He, therefore, relegated these as- 
sumed tastes to olfaction. 

Insipidity, such as is characteristic of distilled water, 
is probably real tastelessness. Oehrwall (1891) attributed 
it to the absence of small amounts of carbon dioxide from 
such waters and this is probably true, for tastelessness 



136 SMELL, TASTE, ALLIED SENSES 

disappears on the addition of some of this gas to insipid 
water. Henle (1880) showed that insipidity was char- 
acteristic of fluids that contained less salt than the saliva. 
Insipidity is probably a deficiency phenomenon and may 
be produced by the absence of several classes of sub- 
stances. Nevertheless it must not be forgotten that a 
condition of staleness or flatness in water, practically 
indistinguishable from insipidity, can be produced by 
introducing into the water very small amounts of caustic 
alkali whereby hydroxyl ions are liberated (Kiesow, 
1894-1896). 

4. The Sour Taste. Sour taste has long been asso- 
ciated with acid substances. In fact it seems very prob- 
able that the sour taste is excited only by acids, acid 
salts, or materials that produce acids. All these sub- 
stances on going into aqueous solution give rise to hydro- 
gen ions by the dissociation of acid molecules. If the 
solutions are strong they will also contain a certain 
number of undissociated acid molecules. It was pointed 
out by Richards (1898) that, since all such solutions have 
the sour taste and since the one component that they all 
have in common is the hydrogen ion, this ion must be the 
occasion of their common taste. This conclusion was 
independently arrived at in another way by Kahlenberg 
(1898). A 0.0025 molar solution of hydrochloric acid has 
a pronounced sour taste and its dissociation into hydro- 
gen and chlorine ions is practically complete. A corres- 
ponding solution of sodium chloride is also about 
completely dissociated into sodium and chlorine ions but 
is without taste. It follows, therefore, since there are 
as many chlorine ions in the salt solution as in the acid 
solution per unit volume and the salt solution is without 



PHYSIOLOGY OF GUSTATION 137 

taste, that the sour taste of the acid solution cannot be 
due to its chlorine ions but must be occasioned by its only 
other constituent, the hydrogen ions. Kahlenberg, there- 
fore, concluded that these ions are accountable for the 
sour taste. 

This view is supported by the fact that the sourness 
of all acid solutions is the same, for instance, it is impos- 
sible to distinguish by taste hydrochloric acid from nitric 
or sulphuric acid. So far as the sensations are concerned 
all these reagents produce identical results, the one qual- 
ity of sourness. There has been some tendency to sepa- 
rate astringency from sourness, but it is generally 
conceded that astringency is merely sourness near the 
vanishing point. With hydrochloric and other mineral 
acids this occurs in molar solutions at about 0.00125 to 
0.001 below which the acid solutions cannot be distin- 
guished from pure water. 

From this standpoint sour taste might be regarded 
as due directly to hydrogen ions and the intensity of this 
taste to depend upon the concentration of such ions. 
But the question is not so simple as this. Although solu- 
tions of most mineral acids agree well among themselves 
so far as sourness and hydrogen ion concentration are 
concerned, organic acids are not necessarily so related. 
Most organic acids are much less dissociated in aqueous 
solution than are inorganic acids and contain, therefore, 
in normal solution, fewer hydrogen ions per unit volume, 
than inorganic acids do. Nevertheless Eichards (1898) 
found that tartaric, citric, and especially acetic acids were 
more sour than would have been expected from the hydro- 
gen ion concentration of their solutions. According to 
Richards acetic acid is about as sour as a solution of 



138 SMELL, TASTE, ALLIED SENSES 

hydrochloric acid one-third as concentrated. Nevertheless 
the acetic acid is dissociated only about one-fourteenth as 
much as the hydrochloric. Hence ion for ion the acetic 
acid solution is the more sour of the two. This result 
was also arrived at by Kahlenbcrg (1898) who estimated 
the sourness of acetic acid at a concentration of 0.005 
molar to be about four times what should be expected from 
its hydrogen ion content. These differences were sub- 
sequently reaffirmed by Becker and Hertzog (1907). 

It is by no means easy to explain the excess of sour- 
ness on the part of acetic and other like acids. Richards 
has suggested, without putting great stress on the idea, 
that the additional sourness of acetic acid may be due to 
the undissociated molecules, which, serving as a reserve, 
producing additional hydrogen ions as those present are 
used up in the reaction between the acid solution and the 
surface of the receptor, an opinion supported by the 
recent work of Harvey (1920). Crozier (1916, 1918a, 
1918b), on the other hand, has pointed out the probability 
that the question is double, one part having to do with 
penetration and the other with the production of the 
sour taste. By taking advantage of natural indica- 
tors, such as the blue pigment in the integument of 
Chrompdoris, it can be easily shown that acids pene- 
trate living cells. This may be assumed to be the 
first step in sour gustation. But penetration observed 
in this way is a much slower process than gustation, 
hence the penetration concerned with taste can have 
to do only with the most superficial layer of the 
taste cells. It is the ease of combination with this layer 
that may make the difference between acetic acid and 
other acids. Different acids having penetrated the sur- 



PHYSIOLOGY OF GUSTATION 139 

faces of gustatory cells at different rates, their uniform 
sour taste may then be ascribed to their common dissocia- 
tion product, the hydrogen ion. How this is accomplished 
is, according to Crozier, the second problem in gustation. 
That the sour taste is in some way dependent upon hydro- 
gen ions seems true beyond reasonable doubt. How these 
ions become effective is still a problem. 

5. The Saline Taste. The saline taste is typified by 
that of common salt. Sodium chloride, however, is not 
the only substance that possesses this taste, for there is 
a whole range of compounds that have the same property. 
The chlorides of potassium, lithium, ammonium, and mag- 
nesium, the hydro chlorides of monomethylamine and of 
diethylamine, the bromides and iodides of sodium and of 
potassium as well as their sulphates and nitrates are 
all more or less saline in taste. 

Aqueous solutions of most of these salts show a high 
degree of dissociation so that, beside undissociated mole- 
cules, cations and anions are present in these solutions 
as possible stimuli for the saline taste. Hober and Kiesow 
(1898) have worked on this question and have declared 
in favor of ions as the stimulating agents in contrast with 
undissociated molecules. Kahlenberg (1898) arrived at 
the same conclusion. He found that a solution of sodium 
chloride, 0.02 molar, was scarcely to be distinguished by 
taste from pure water. At 0.04 molar it was a trifle 
saline. Corresponding solutions of sodium acetate were 
almost tasteless and certainly not in the least saline. 
Hence it is evident that the salty taste of sodium chloride 
is due to chlorine ions and not to sodium ions. This con- 
clusion is supported by the fact that 0.04 molar solutions 
of potassium chloride and of lithium chloride are also 



140 SMELL, TASTE, ALLIED SENSES 

salty. Other chlorides, such as those of ammonium and 
magnesium, have a saline taste. 

This taste, however, is not due exclusively to chlorine 
ions. Sodium bromide at 0.02 molar has a faint saline 
taste and is unquestionably salty at 0.04. Hence the 
bromine ion must also be a stimulus for the salty taste. 
Kahlenberg (1898) reported it as not quite so effective 
in this respect as the chlorine ion. Although solutions 
of sodium iodide at 0.04 or even at 0.02 molar could be 
distinguished from water, they did not give an unques- 
tionable taste till a concentration of 0.16 was reached. 
At this concentration the taste w r as markedly saline. A 
corresponding solution of potassium iodide was found 
also to be salty though in this instance the taste was ac- 
companied by a slightly bitter flavor. Prom these con- 
siderations it is evident that iodine ions are saline stimuli 
though they are not so effective in this respect as chlorine 
or bromine ions are. The sulphates of sodium and of 
potassium as w r ell as their nitrates also have a saltiness 
in their tastes and it has been shown in these instances 
that the sulphate and nitrate ions are the effective agents. 
Thus all saline tastes depend upon ionic stimuli, and, as 
Kahlenberg (1898) and Hober and Kiesow (1898) have 
maintained, these ions are always anions, a conclusion 
supported by the more recent work of Herlitzka (1908). 

6. The Bitter Taste. The bitter taste is character- 
istic of almost all alkaloids, and of certain unrelated sub- 
stances such as dextro-mannose, the glucosides, picric 
acid, ether, and certain inorganic salts such as magnesium 
sulphate or Epsom salt. 

Magnesium salts when sufficiently concentrated have 
a bitter taste and this taste is due to the magnesium ion. 
This is in strong contrast with the ions of sodium and 



PHYSIOLOGY OF GUSTATION 141 

of lithium, which are apparently almost tasteless. Am- 
monium and calcium ions are also bitter in taste. In 
picric acid the sour taste of the hydrogen ion is probably 
completely masked by the bitter taste of the picric anion 
though the taste of this substance as well as that of ether, 
dextro-mannose, the glucosides and other such substances 
appears never to have been fully investigated. 

But the substances that are especially characterized 
by bitter tastes are the alkaloids. These include such 
compounds as morphine, cocaine, pilocarpine, quinine, 
nicotine, and strychnine, the bitterest of all substances. 
In aqueous solution these substances are the most effec- 
tive agents in exciting the bitter taste. Gley and Richet 
(1885) determined that strychnine monochloride could be 
tasted at 0.0006 gram per liter of water. Of such a 
solution 5 cubic centimeters, which was the volume used 
by these investigators in their individual tests, contains 
only 0.000005 gram of the bitter material and yet this very 
small amount produces a pronounced taste. Quinine 
hydrochloride can be tasted in a solution as dilute as 
0.00004 molar (Parker and Stabler, 1913). Thus bitter 
substances far exceed hydrogen ions in their capacity to 
stimulate at high dilution. 

What peculiar chemical feature is characteristic of 
bitter organic substances whereby they excite this taste 
is at best, poorly understood. Henry (1895) pointed out 
that the bitter compounds often included the group 

/CH 2 OH 
N0 2 -C- 

\ 

and this was confirmed by Cohn (1914) whose extensive 
study of the sapid organic compounds led him to the con- 
clusion that there were several such groups, the presence 



142 SMELL, TASTE, ALLIED SENSES 

of any one of which in a given compound would give it 
a bitter taste. In dyes color-radicals have long been 
called chromophores ; by analogy radicals concerned with 
taste have been designated saprophores. Among these 
are hydroxyl and the amine group. The nitro group N0 2 
is often associated, especially in aromatic compounds, 
with a bitter taste. When three N0 2 groups are included 
in a given compound, it always has a bitter taste ; when 
two are at hand, the taste is commonly bitter but not 
invariably so; when only one such group is present, the 
taste is not bitter. Thus the number of N0 2 groups ap- 
pears to be significant in the production of a bitter taste. 

The bitter taste, then, is excited by several classes of 
substances; by ions that, with the possible exception of 
the anion of picric acid, are apparently always cations 
Herlitzka (1908), magnesium, ammonium, and calcium; 
and by organic substances, especially the alkaloids, which 
may act either through their molecules or through certain 
atomic groups, the so-called saprophores. 

7. The Sweet Taste. The sweet taste is excited by 
the diatomic and polyatomic alcohols of the aliphatic 
series, by the aldehydes and ketons derived from these 
alcohols, and especially by the hexoses whose polymeriza- 
tion products, the disaccharides arid polysaccharides, are 
in this respect particularly important. Besides these 
carbohydrates other organic compounds, such as chloro- 
form, dextro-asparagine, and saccharine, have sweet 
tastes. Among inorganic substances neutral acetate of 
lead, often called sugar of lead, and the salts of glucinum 
are known to be sweet. Solutions of the alkalis, if they 
are of appropriate dilution, are said likewise to excite 
this taste. 



PHYSIOLOGY OF GUSTATION 143 

What occasions the sweet taste of lead acetate seems 
never to have been ascertained. On the other hand glu- 
cinum chloride and glucinum sulphate, both of which 
break into ions in water, have been shown by Hober and 
Kiesow (1898) to owe their sweet taste to their common 
constituent, the glucinum ion. Thus ions are one means 
of exciting this taste. 

But the sweet taste, like the bitter one, is primarily 
associated with organic compounds. It centers about the 
alcohols and especially the sugars in much the same way 
that the bitter taste does about the alkaloids. Although 
the halogenated hydrocarbon chloroform and the aromatic 
compound saccharine are both sweet, the latter about 500 
times as much so as cane sugar, the great majority of sweet 
substances are aliphatic alcohols and their derivatives. 
Ethyl alcohol is sweetish in taste as well as the trihydric 
alcohol glycerol, but the type of sweet substances is cane 
sugar or sucrose. This can be tasted in aqueous solution 
to about 0.02 molar; in weaker concentrations it is diffi- 
cult to distinguish it from pure water. Ethyl alcohol 
cannot be tasted in solutions much weaker than 3 molar, a 
relatively high concentration (Parker and Stabler, 1913). 

What determines the sweet taste in carbohydrates is 
by no means settled. It apparently turns upon very 
slight differences. These are sometimes sterioisomeric 
in character. Thus, as already stated, dextro-asparagine 
is sweet and laevo-asparagine is tasteless. Dextro-man- 
nose is sweet and its stereoisomer dextro-glucose is bitter. 
Other such examples are known. In some instances 
slight changes in composition are accompanied by con- 
siderable changes in taste. Thus, according to Thorns 
and Nettesheim (1920), dulcin loses its sweetening power 



144 SMELL, TASTE, ALLIED SENSES 

when acidic or basic substitutes are introduced into its 
benzene nucleus. The introduction into a sweet molecule 
of any considerable radical, especially an aromatic 
one, is very likely to be followed by a change from 
sweet to bitter. 

Colin (1914) made an elaborate comparison of the 
constitution of the sweet substances, as he did that of 
the bitter compounds, and came to the conclusion that 
these substances like the bitter ones contained particular 
groups of atoms that determined their taste and that he 
designated as glucogenes. Thus among alcohols one hy- 
droxyl is accompanied with slight sweetness and four 
or five with intense sweetness. But notwithstanding 
the extent of Colin 's comparisons, Oertly and Myers 
(1919) found his generalizations inadequate, and pro- 
posed in place of his hypothesis one in which two groups 
were assumed to be present in every sweet molecule. Fol- 
lowing by analogy the terminology used for dyes, one of 
these groups was called a glucophore and the other an 
auxogluc. By a close comparison of the sugars, amino 
acids, and halogen derivatives of the hydrocarbons, they 
believed they could identify at least six glucophores and 
nine auxoglucs. The glucophores are (1) CH 2 OH-CHOH-, 
(2) -CO-CHOH-(H), (3) C0 2 H-CHNH 2 -, (4) -CH 2 ON0 2 , 

(5) CJ-*-, and WC^-'-C^-. The auxoglucs are (1) 

H-, (2) X CH,-, (3) CH 3 CH 2 -* (4) CH,-CH 2 -CH 2 -, (5) 
(CH 3 ) 2 CH-, (6) CH,OH-, (7) CH,CHOH-, (8) CH 2 0!H- 
CH 2 -, and (9) radicals C^Ho^On of normal polyhe- 
dric alcohols. 

An illustration of the way in which Oertly and Myers' 
theory may be made to apply to sweet substances is given 



PHYSIOLOGY OF GUSTATION 145 

in the following table in which the resolution of a number 
of sweet compounds into glucophores and auxoglucs 
is indicated. 

Table VII. 

A table of sweet organic compounds (aliphatic series) showing the 
constitution of the compound and its resolution into a glucophore and an 
auxogluc, from Oertly and Myers (1919). 

Name of 

Compound Constitution Glucophore Auxogluc 

Glycol CH 2 OH-CH 2 OH CH 2 OH-CHOH H- 

Glycerol CH 2 OH-CHOH-CH 2 OH CH 2 OH-CHOH CH 2 OH- 

Fructose CH 2 OHCO ( CHOH ) 8 CH 2 OH .COCHOH-(H) C n H 2n +iOn- 

Glycine CH 2 NH 2 -COOH -CHNH 2 -COOH H- 

Ethyl nitrate C 2 H 3 ON0 2 -CH 2 ON0 2 CH 3 - 

Notwithstanding the elaborate attempts of Cohn and 
of Oertly and Myers to elucidate the chemoreception of 
sweet substances, the subject must be admitted to be one 
that is far from settled. What may be said with cer- 
tainty is that the sweet taste, like the bitter taste, is ex- 
cited both by ions and by organic molecules the details 
of whose activity, however, are by no means fully 
worked out. 

8. Inadequate Stimuli. Taste is somewhat remark- 
able for its paucity of inadequate stimuli. Although the 
tongue is very sensitive to temperature differences, these 
changes do not seem to excite the gustatory receptors. 
It is questionable whether mechanical stimulation, such 
as tapping the tongue as practised by the older physiolo- 
gists, will call forth sensations of taste. The only really 
effective form of inadequate stimulus for the gustatory 
organs seems to be the electric current. As early as 
1752 Sulzer noted the peculiar sensations when two dif- 
10 



146 SMELL, TASTE, ALLIED SENSES 

ferent metals are placed simultaneously on the tongue. 
This observation was independently made by Volta in 
1792 who believed these sensations to be produced by the 
electrical stimulation of the organs of taste, for he ob- 
tained the same effects by passing an electric current 
through the tongue. Five years later, however, Humboldt 
pointed out that the real stimulating agent in the 
so-called electric taste might be the substances produced 
by electrolysis at the region where the current passes 
from the electrode into the tongue rather than the electric 
current itself. Thus was established the two opposing 
views concerning electrical taste. 

If an electric current is passed through the human 
body in such a way that the anode is applied to the tongue 
and the cathode to some other part, a sour taste develops 
around the anode. If the electrodes are reversed in posi- 
tion, an alkaline taste appears at the cathode. This con- 
forms with what takes place when an electric current is 
passed through an alkaline solution, such as the saliva; 
hydrogen ions appear at the anode and hydroxyl ions 
at the cathode. Why then are not these two substances, 
the hydrogen and the hydroxyl, the stimuli for the char- 
acteristic tastes? 

But Rosenthal (1860) and, before him, Volta, found 
that if the anode is a weak alkaline solution into which 
the tip of the tongue is dipped, a sour taste nevertheless 
arises, though the hydrogen ions under such a combination 
might be expected to be neutralized immediately by the 
hydroxyl present. Rosenthal also showed that if an elec- 
tric current is passed through the bodies of two persons 
and is completed by bringing the tip of the tongue of one 
of these individuals into contact with that of the other, 



PHYSIOLOGY OF GUSTATION 147 

the two persons experience different sensations, one sour 
and the other alkaline. These and other like experi- 
ments led Eosenthal to conclude that the electric current 
itself was the stimulating agent and not the materials 
produced by electrolysis. 

But it must not be forgotten that the electrical stimu- 
lation of organs of taste is productive of a variety of 
sensations. Thus in 1798 Ritter showed that after a cur- 
rent had been passing for some time through the tongue 
the sour taste of the anode changed first to bitter and then 
to alkaline while the cathodic alkaline taste changed to 
sour. Hofmann and Bunzel (1897) demonstrated that 
during the passage of a current there is at the cathode 
a burning bitter sensation which changes to a sour metal- 
lic taste on breaking the current. The initial taste they 
believed to be due to the products of electrolysis. Von 
Zeynek (1898) also accepted this explanation for the elec- 
tric taste, Gertz (1919), however, pointed out that the 
alternating current is really more effective in exciting 
taste than the direct current and that hence the electric 
taste may be aroused by other means than the products 
of electrolysis. It is not at all impossible that the gusta- 
tory organs may be excited in both ways : by the materials 
of electrolytic decomposition and directly by the electric 
current. But how an electric current can stimulate gus- 
tation without in some way bringing about a chemical 
change, at least within the gustatory cell, is difficult 
to imagine. 

The extreme sensitiveness of the organs of taste to 
electrical stimulation is not only characteristic of man, 
but is probably found throughout the vertebrates. 
Among fishes the catfish or horned pout, Amiurus, is ap- 



148 SMELL, TASTE, ALLIED SENSES 

parently easily stimulated in this way. The head and 
especially the eight barbels about the mouth of Amiurus 
are richly supplied with taste-buds. These organs, like 
those on the human tongue, are apparently extremely 
sensitive to metals probably because of the slight electric 
currents produced by these bodies, for, the fishes respond 
with great readiness to a weak constant current from a 
dry cell. If such a current is led into an aquarium 
through a water-filled glass tube and out again by a sim- 
ilar tube, the water acting as a conductor, catfishes can 
be readily stimulated by bringing such tubes close to 
them. If the current is sufficiently reduced (a little less 
than a microampere) the fishes will approach the open 
ends of the tubes and nibble at the current as though it 
were a bait, thus giving evidence that the organs stimu- 
lated are the gustatory receptors (Parker and Van 
Heusen, 1917). Hence the electric stimulus seems in every 
way to duplicate the stimulus normal for the organ of 
taste, a solution of a sapid substance. 

9. Distribution of Tastes on the Tongue. The four 
well-recognized tastes, as the preceding sections show, 
are normally excited by very different stimuli. The sour 
taste is dependent upon the cation, hydrogen. The saline 
taste is called forth by a number of anions : chlorine, 
bromine, iodine, and the sulphate and nitrate ions. The 
bitter taste has as stimuli the alkaloids, such cations as 
magnesium, ammonium, and calcium, and possibly the 
anion of picric acid. The sweet taste depends upon such 
organic compounds as the sugars and alcohols, and on 
saccharine, on lead acetate, and on hydroxyl and gluci- 
num ions. The four tastes, therefore, are excited by 
entirely independent groups of stimuli and it is a matter 



PHYSIOLOGY OF GUSTATION 



149 



of importance to ascertain in what other respects they 
are independent. This question can be well approached 
from the standpoint of their distribution on the tongue. 
As already mentioned, the tongue of the normal adult 
human being is only in part gustatory, its lower surface 
and the central portion of its upper surface being 








A B C D 

FIG. 36. Diagrams of the right half of the human tongue illustrating the distribution of 
the four tastes; the dots represent the area and concentration of a given taste: A, the sour 
taste, concentrated on the edge; B, the saline taste, concentrated at the tip and on the edge; 
C, the bitter taste, concentrated at the base; D, the sweet taste, concentrated at the tip. 
Modified from Hanig, 1901. 

devoid of taste. This sense is resident only on the 
tip, the edges and the dorsal part of the root of the 
tongue. The distribution of the several tastes over the 
gustatory portion of the tongue has been a matter of 
investigation for physiologists during more than a cen- 
tury, and the results, particularly among the recent 
workers, have been remarkably consistent and harmoni- 
ous. Shore (1892), Kiesow (1894-1896), and Hanig 
(1901) have been the most important recent contributors 



150 SMELL, TASTE, ALLIED SENSES 

to this subject. Their work shows that the four tastes 
have decidedly individual distributions on the tongue. 
The sour taste is best developed on the lateral edges of 
the tongue and diminishes from these regions toward the 
tip, the base, and the central anaesthetic area (Fig. 36, A). 
The saline taste is most pronounced at the tip and on 
the lateral margins of the tongue and diminishes at the 
base; toward the central area it ends rather abruptly 
(Fig. 36, B). The bitter taste is most characteristic of the 
base of the tongue especially in the region of the vallate pa- 
pillae whence it diminishes rapidly toward the central area 
and over the lateral edges to the tip (Fig. 36,C). The sweet 
taste is at its maximum at the tip of the tongue and di- 
minishes thence along the lateral margins to the base (Fig. 
36, D). Thus sour is represented by two marginal re- 
gions, saline by a horse-shoe shaped area at the tip, bit- 
ter by a single center at the base, and sweet by one at 
the tip. It is difficult to explain these differences in the 
distribution of the tastes except on the assumption of 
an independent sensory mechanism for each taste. 

This interpretation of taste is strengthened by what 
has been learned from the local stimulation of the tongue. 
Oehrwall (1891) mapped out a group of fungiform papillae 
near the tip of the tongue in such a way that the 
individual papillae could be reidentified and studied. 
Each papilla was stimulated by applying to it the point 
of a very fine brush loaded with a strong solution of a 
given substance. The substances used were tartaric 
acid 2 per cent, common salt 20 per cent, quinine hydro- 
chloride 2 per cent, and sugar 40 per cent. The salt was 
finally abandoned because of the indistinctness of the 
sensation. In all 125 easily identifiable papillae were 



PHYSIOLOGY OF GUSTATION 151 

tested. All of these were found to be sensitive to touch, 
warmth, and cold, but only 98 were stimulated by the 
solutions used. The results of these tests so far as they 
relate to taste are given in the following tabulation. 

Test Substances Acid Quinine Sugar 

Number of papillse sensitive 91 71 79 

Number exclusively sensitive! 12 3 

The fact that 12 papillae were stimulated by tartaric 
acid but not by quinine or sugar and that 3 were stimu- 
lated by sugar but not by tartaric acid or quinine is strong 
evidence in favor of the independence of at least the sour 
and sweet tastes. Oehrwall also discovered by his 
method of local stimulation that the surface of the tongue 
between the papillae was insensitive to taste. 

This result confirmed the earlier work of Goldscheider 
and Schmidt (1890) who had also shown that when pa- 
pillae were tested with a mixed solution of sugar and 
quinine sometimes a sweet taste was evoked and at other 
times a bitter one. 

This whole subject was thoroughly re-investigated by 
Kiesow (1898), who used as stimuli solutions of hydro- 
chloric acid, of sodium chloride, of quinine sulphate and 
of sugar. Of the 39 papillae tested 4 were found to be 
insensitive. The conditions presented by the remaining 
35 are summarized in the following tabulation. 

Test Substances Acid Salt Quinine Sugar 

Number of papillae sensitive 18 18 13 26 

Number exclusively sensitive 3 3 7 

Failed of stimulation 17 17 22 9 

These results confirm and extend the original findings 
of Oehrwall in that they show the independence of the 
sour, saline, and sweet tastes. The fact that the region 



152 SMELL, TASTE, ALLIED SENSES 

tested was near the tip of the tongue is probably the 
occasion of the absence in the records of any papillae 
stimulated exclusively by quinine, for this region is one 
in which the bitter taste is least developed. Kiesow also 
observed that the papillae presented a great variety of 
combinations in taste ; some were open to stimulation by 
two of the four reagents used, others by three and still 
others by all four. It is known that each gustatory 
papilla carries a number of taste-buds but whether in 
those papillae that are open to stimulation by two or more 
sapid solutions there is a corresponding number of kinds 
of buds, one for acid, another for salt reception and so 
forth, cannot be stated, for it is possible that this dif- 
ferentiation may reach to the gustatory cells of each 
bud. What can be affirmed, however, is that in those 
papillae that respond exclusively to one taste all taste- 
buds with their contained cells must be so constituted as 
to be open to stimulation by one class of sapid substances 
and to be closed to all other classes. Thus in a papilla 
that is stimulated exclusively by acid the protoplasm of 
the receptive cells in all its taste-buds must be organized 
to receive acid stimuli and not to react to those for the 
saline, bitter and sweet tastes. This conclusion amounts 
to a very complete confirmation of Miiller's theory of the 
specific energy of sensory nerves as it is interpreted in 
modern physiology and to the component theory as ap- 
plied to taste. 

10. Action of Drugs on Taste. Certain drugs have 
the remarkable property of temporarily diminishing or 
even obliterating taste. Edgeworth discovered that af- 
ter a person had masticated the leaves of the Indian 
asclepiad Gymnema sylvestre, he was unable to taste 



PHYSIOLOGY OF GUSTATION 153 

sugar. Hooper (1887) extracted from the leaves of this 
plant a compound that he named gymnemic acid and that 
he showed to be the substance that affected taste. Ac- 
cording to him gymnemic acid tends to obliterate the 
sweet and bitter tastes but has no effect on the saline and 
sour tastes. Shore (1892) studied the influence of gym- 
nema decoctions on the tongue and found that they oblit- 
erated the sweet taste of glycerine very easily and the 
bitter taste of quinine almost as readily. They had very 
little effect on the taste of sulphuric acid or of common salt. 
These results were confirmed in the main by Kiesow 
(1894). Thus gymnemic acid divides the tastes into at 
least two distinct classes, one including sweet and bitter, 
and the other sour and saline. 

Stovaine is also known to abolish sweet and bitter 
without obliterating saline and sour (Ponzo, 1909) and 
eucaine-B especially reduces bitter (Fontana, 1902). 
Saline and sweet tastes and in less degree bitter are 
reduced by a 0.02 normal solution of chromium nitrate 
(Herlitzka, 1909). 

The effect of cocaine on taste is very profound. Von 
Anrep (1880) and Knapp (1884) observed that this nar- 
cotic was capable of abolishing completely all taste. 
Aducco and Mosso (1886) showed, however, that it acted 
more energetically on the bitter taste than on the others. 
Shore (1892) found that on treatment with cocaine the 
buccal sensations were extinguished in a definite order 
as follows : pain, bitter, sweet, saline, sour, and touch, a 
sequence confirmed by Kiesow (1894). Thus cocaine is 
more selective in its effect on taste than gymnemic acid 
and leads to a separation of all four tastes. 

11. Substances with two Tastes. A number of sub- 



154 SMELL, TASTE, ALLIED SENSES 

stances are known that possess different tastes depend- 
ing upon the part of the tongue to which they are applied. 
Many salts have this peculiarity Herlitzka (1908). 
Potassium nitrate and magnesium sulphate are both said 
to be saline in taste when applied at the tip of the tongue 
and bitter at its base. This action, however, is proba- 
bly due not to the molecules of the salts but to their ions. 
At the tip of the tongue the anions stimulate the organs 
of the saline taste, which in this location are in the ascen- 
dency, and at the base of the tongue the cations stimulate 
the organs of the bitter taste which is here better devel- 
oped. There is thus a kind of competition between the 
two sets of ions, as Herlitzka has expressed it, and in one 
locality the anions win out, in the other the cations. 

Such an explanation, however, does not apply to sub- 
stances like parabrombenzoic sulphinide. This material, 
according to Howell and Kastle (1887) has a distinctly 
sweet taste when applied to the tip of the tongue and an 
intensely bitter one at the back. Dulcamarin, the gluco- 
side from bittersweet, is another case of the same kind; a 
list of these is given by Sternberg (1898). In these in- 
stances ions are probably not involved, but each substance 
is a stimulus for both the organs of the sweet taste and 
of the bitter taste. It seems impossible to explain double 
tastes such as those just mentioned except on the assump- 
tion of independent receptor systems for the tastes con- 
cerned. Thus far no substance is known that excites 
three categories of tastes though I know of no reason why 
such a substance might not exist. 

12. Latency of Taste Sensations. Von "Wittich 
(1868) appears to have been the first to attempt to meas- 
ure the interval of time between the application of a 



PHYSIOLOGY OF GUSTATION 155 

stimulus to a gustatory portion of the tongue and the 
response of the subject. He used an electric current as a 
stimulus and found the average time to be 0.167 seconds. 
Von Vintschgau and Honigschniied (1875-1877), who used 
solutions of various substances as stimuli, found that the 
times were different for the different tastes, being short- 
est for saline, longer for sweet, still longer for sour and 
longest for bitter. They also discovered that the times 
were different for the tip of the tongue and its base. 
Their results were confirmed in general by the later in- 
vestigations of Beaunis (1884), of Henry (1895) and of 
Kiesow (1903) who recorded the following periods for 
the tip of the tongue : 

Sodium chloride 0.308 second 

Sugar 0,446 second 

Hydrochloric acid 0.536 second 

Quinine 1.082 second 

These records agree with Schirmer's observation 
(1859) that when a solution containing all four sapid sub- 
stances is placed on the tongue, the subject experiences 
the sensations in the order saline, sweet, sour, and bitter. 
They also confirm the opinion that the four tastes are 
separate entities. 

One aspect of the problem of gustatory latency turns 
on temperature. If the stimulation of a taste receptor 
is a chemical operation, this process should exhibit a con- 
siderable temperature co-efficient that might make itself 
felt in a change in the latent period. But so far as I am 
aware no studies with this point in view have been car- 
ried out. 

13. Taste Alterations; After-tastes. A number of 



156 SMELL, TASTE, ALLIED SENSES 

substances are known whose solutions so affect the tongue 
that its powers of taste become temporarily changed. 
Thus these substances give rise to what have been called 
after-tastes. In almost every instance the taste that 
suffers change is the sweet taste and this is increased in 
efficiency. Thus Aducco and Mosso (1886) found that 
after the tongue had been held in dilute sulphuric acid 
for five to ten minutes, distilled water was then capable 
of exciting a very sweet taste. A solution of quinine was 
also sweet to the taste at the tip of the tongue, but it 
remained normally bitter at the base. This change was 
not brought about by other acids such as acetic, citric, and 
formic. Frentzel (1896) also noticed that after washing 
out the mouth with a weak solution of copper sulphate, 
smoking a cigar was accompanied by a sweet taste. Ac- 
cording to Zuntz (1892) a solution of sodium chloride of 
one per cent strength will increase the sweetness of sugar, 
an observation confirmed by Heymans (1899). A mouth 
wash of potassium chlorate is well known, to leave the 
tongue so that distilled water tastes sweet (Nagel, 1896). 
In all these instances it is probable that the constitution 
of the receptor for the sweet taste is so changed by the 
first solution applied to it that it becomes hypersensitive 
to its normal stimuli such as sugar or even open to novel 
stimuli such as distilled water. 

Complete loss of taste or ageusia is known to accom- 
pany hysterical and other abnormal nervous states. It 
may be temporary or, in the case of certain lesions, per- 
manent in character. 

14. Gustatory Contrasts. Although some acids in- 
crease the sensitiveness of the sweet taste and thus give 
ground for a gustatory contrast, it is questionable whether 



PHYSIOLOGY OF GUSTATION 157 

such contrasts exist as extensively as was believed by the 
older workers. It is a common opinion that after a sweet 
drink a sour taste is more intense, but Oehrwall (1891) 
was unable to confirm this experimentally nor could he 
show that bitter increased the sensibility to sweet. 
Haycraft (1900) noted that when one border of the tongue 
is rubbed with salt, the other border becomes hypersensi- 
tive to sugar, but such a contrast is clearly not peripheral 
but central in origin, and possibly other contrasts may 
be thus explained. 

15. Taste Compensations and Mixtures. Mixtures 
of sapid solutions do not as a rule give rise to tastes other 
than those of their components. Lemonade has both the 
sweet taste of the sugar and the sour taste of the citric 
acid it contains. Sugar adds a pleasant element to cof- 
fee, but does not destroy its bitter taste. In ordinary 
food the flavor is the mixture of true tastes and odors 
accompanied by the multitude of other buccal sensitivities 
due to the variety of substances in the mouth and accep- 
ted in a rather unanalyzed form by the central apparatus. 
Yet in all this complexity the elements remain essentially 
distinct. Competition rather than compensation seems 
to be the rule. Kiesow (1894-1896) , however, has claimed 
that a very weak solution of sugar and salt gives a taste 
that is neither sweet nor saline but distinctly flat, and 
Kremer (1918) has recently shown that a solution of 
sodium chloride too weak to stimulate the saline taste 
will, nevertheless, considerably increase the sweetness of 
a cane-sugar solution. Quinine hydrochloride on the 
other hand will, according to Kremer, reduce sweetness. 
These instances may be evidence of gustatory compensa- 
tion, but it seems much more probable, as was indicated 



158 SMELL, TASTE, ALLIED SENSES 

in a preceding section, that they result from a sensitizing 
or a desensitizing of the sweet receptors by the sodium 
chloride or the quinine, for it is extremely doubtful, as 
Oehrwall (1891) has stated, whether true gustatory com- 
pensation ever occurs. Ionic antagonism such as Crozier 
(1915) has discovered in the reaction of the frog's foot to 
salt solution has thus far not been identified in taste. 

16. The Gustatory Senses. When a general survey 
of the so-called sense of taste is made, the most striking 
feature that appears is the remarkable independence of 
the four categories, sour, saline, bitter, and sweet. These 
are excited by groups of different stimuli, they give re- 
markable evidence of having separate receptors, they are 
differently acted upon by various drugs, and they show 
numerous other peculiarities that are interpretable only 
from the standpoint of organic separateness. So im- 
pressed was Oehrwall (1891, 1901) with these peculiari- 
ties that he declared them to be in all essentials four 
separate senses, a declaration entirely in accord with the 
component theory as applied to taste. Although this 
view has a certain radical element in it and has not been 
favorably received by such workers as Kiesow, Nagel, 
Luciani, and Henning, who have declared for the unitary 
nature of taste, it is difficult to say why it should not pre- 
vail. It has been urged that gustatory compensation is 
inconsistent with Oehrwall 's hypothesis and possibly this 
may be true. But gustatory compensation is so uncer- 
tain a phenomenon that when compared with the sub- 
stantial body of evidence in favor of the hypothesis, this 
objection lacks force. Henning (1916) has declared that 
the tastes of different substances, members of one cate- 
gory, are not necessarily alike; thus the saline tastes of 



PHYSIOLOGY OF GUSTATION 159 

sodium chloride, sodium iodide, and sodium bromide, 
though much the same are still characteristically different. 
And he has further maintained that the mixed tastes so- 
called cannot be imitated by real mixtures; thus the 
bitter-saline taste of magnesium chloride cannot be repro- 
duced, he believes, by a mixture of sodium chloride and 
bitter aloes. But all such statements imply that the 
conception of the receptive independence of tastes neces- 
sarily involves the further view that a gustatory stimulus 
is limited to one category of receptors. That some sub- 
stances, such as parabrombenzoic sulphinide, stimulate 
two categories of receptors has already been made clear 
and though most stimulating materials influence in a 
vigorous way only one set of end-organs, it is more than 
probable that they all affect at least to a slight degree 
other such sets. The taste of any substance then is not 
necessarily one of the four tastes and this alone, but one 
of these qualified by traces of other tastes excited slightly 
and simultaneously by the same stimulating agent. 
Hence any substance such as sodium chloride, or sodium 
bromide, may perfectly well have a somewhat individual 
taste without doing violence to the hypothesis that there 
are four separate tastes, and the success with which 
mixed tastes so-called may be imitated is rather a matter 
of skill than despair. 

It is time that gustation is a strikingly unified oper- 
ation, but when this unity is looked into, it is seen to 
depend upon simultaneousness of action rather than on 
interdependence of activities. Smell is related to taste 
in much the same way that one taste is related to another. 
On the whole it would seem more consistent with fact to 
speak of the sour sense, the saline, the sweet, and the 



160 SMELL, TASTE, ALLIED SENSES 

bitter sense than of the sense of taste. Just as the sense 
of feeling in the skin has been shown to consist of at 
least four senses, touch, pain, heat, and cold, so taste must 
be regarded as composed of at least four senses. That 
these act together and in everyday experience produce a 
unified effect upon us is no more reason for classing them 
as one sense than in the case of the integumentary senses. 
The sense of taste must, therefore, be regarded as a ge- 
neric term under which at least four true senses are gath- 
ered: sour, saline, bitter, and sweet (Oehrwall, 1891, 1901). 

Although the sense of taste thus loses a certain amount 
of its reality, the senses classed under it probably possess 
a kind of genetic unity that is not without significance. 
It is very probable that these four senses represent four 
lines of differentiation that have evolved from a single 
ancestral sense. The remarkable uniformity of their 
structure is suggestive of this view. If the four senses 
under discussion have had some such origin as that just 
indicated, the term sense of taste might well apply to 
that primitive state, perhaps represented in some of the 
lower vertebrates today, from which the four gustatory 
senses of man have been derived. 

17. Comparative. The comparative physiology of 
taste in vertebrates is almost an untouched field. The 
distribution of taste-buds in the vertebrate classes indi- 
cates the presence of this sense in the mouth regions in 
forms as low as the amphibians. In fishes Herrick ( 1903) 
lists over thirty -five species in which taste-buds are known 
to occur on the outer surface of the animal as well as 
in the mouth. The catfish Amiurus is remarkable in this 
respect in that its whole outer surface is provided with 
these organs which are most abundantly present on the 



PHYSIOLOGY OF GUSTATION 161 

barbels. When a piece of meat is brought into contact 
with the barbel of one of these fishes, the animal will 
immediately seize and swallow the morsel. The same is 
true when the meat is brought in contact with the side of 
the fish. This quick seizure and swallowing of the food 
has been called by Herrick the gustatory response. If 
a barbel or the flank of Amiurus is touched with a pledget 
of cotton instead of the meat, the fish will turn toward 
the object, but, as a rule, will not snap at it. This Herrick 
has designated the tactile response. If, now, the 
cotton is soaked with meat juice and brought to the side 
of the fish, the quick gustatory response follows. The 
same form of response is made to meat juice discharged 
from a pipette on the side of the fish. From this and 
other tests Herrick concluded that the gustatory response 
in Amiurus could be called forth by purely gustatory 
stimuli unaccompanied by touch and that for this fish 
taste is accompanied by a local sign as touch is. That 
these responses are really gustatory is shown by the fact 
that when the branch of the seventh nerve that innervates 
the taste-buds on the flank of Amiurus is cut, the re- 
sponses no longer occur (Parker, 1912). 

Conditions similar to those in Amiurus were recorded 
by Herrick in a number of gadoid fishes and it is thus 
clear that taste is a general integumentary function in 
many of these animals. To what extent the taste-buds of 
the fish skin are differentiated for the several senses of 
sour, saline, bitter, and sweet cannot be stated. It is 
remarkable, however, that in almost all the fishes tested 
no response to sugar has been found not only on the sur- 
face of the body but also in the mouth (Parker, 1912). 
11 



162 SMELL, TASTE, ALLIED SENSES 

The sweet sense may, therefore, be an exclusive posses- 
sion of the higher vertebrates. 

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CHAPTER VIII. 

INTERKELATION OF THE CHEMICAL SENSES. 

Contents. 1. Common Features in the Stimulation of 
Chemoreceptors. 2. Differences among Chemoreceptors. 
3. Groups of Chemoreceptors. 4. Classification of Recep- 
tors in General. 5. Genetic Relations of Chemoreceptors. 
6. Bibliography. 

1. COMMON Features in the Stimulation of Chemo- 
receptors. The sense organs that have been discussed in 
this volume, the olfactory organs, the vomero-nasal or- 
gans, the common chemical receptors, and the organs of 
taste, form a more or less natural group of organs under 
the general title of chemical receptors or Chemoreceptors. 
This designation is justified by the fact that in stimula- 
tion these several types of receptors present certain 
important features in common. In all instances they 
are activated by solutions. This is most obvious in the 
sense of taste whose stimuli from fishes to mammals con- 
sist of materials in solution either in the water that enters 
the mouth or in the saliva that is mingled with the crushed 
food. An aqueous solution is also the stimulus for the 
common chemical receptors. The nasal cavities of fishes 
are likewise bathed by a continuous stream of water that 
carries the stimulating substances to the olfactory sur- 
faces. And in the air-inhabiting vertebrates, as already 
pointed out, the olfactory terminals are probably not 
exposed in any direct way to the air that carries the stim- 
ulating material but are immersed in mucous through 

167 



168 SMELL, TASTE, ALLIED SENSES 

which this material must make its way before it becomes 
effective. In olfaction, moreover, it is probably not 
simply a question of aqueous solution but, as already ex- 
plained, one of solution in oil as well, for the olfactory 
stimulus seems to be a material that must reach its recep- 
tors through an aqueous medium that covers them and 
then enter them through their lipoid components. What 
has been said of the stimulation of the olfactory organ 
is probably true of the vomero-nasal organ also. Thus 
in one way or another all appropriate stimuli of the so- 
called chemoreceptors are materials in solution. 

But not all soluble materials stimulate the chemorecep- 
tors. Thus such elementary gases as hydrogen, oxygen, 
and nitrogen are odorless and tasteless, and a number of 
organic substances have no stimulating capacity for 
these organs. Those substances that do stimulate, as 
was pointed out especially in the case of taste, fall into 
groups whose characteristics are chemical and not phys- 
ical and, though such an analysis cannot at present be 
made with certainty for smell, it has already been pointed 
out that the variety of smells can be explained only on a 
chemical basis. Thus chemoreceptors are stimulated not 
simply by material in solution, but by the chemical activ- 
ity of dissolved material. On this assumption it is nat- 
ural to expect that there would be a certain number of 
substances, chemically inert toward the given receptors, 
that would, therefore, be incapable of acting as stimuli 
for them. Such substances as the gases already men- 
tioned probably represent this group. 

The stimulus for the chemoreceptor, however, is not 
only a solution of a chemically active material, but it is 
such a solution applied directly to the terminal organ. 



INTERRELATION OF THE CHEMICAL SENSES 169 

This peculiarity of the chemoreceptors is in strong con- 
trast with that which occurs in the so-called mechanicore- 
ceptors, the organs of touch, pressure, and hearing. In 
these organs the appropriate stimulus is a deforming 
pressure which may be exerted by an impinging or vi- 
brating material that does not necessarily touch the 
terminal organ itself, but may act through a considerable 
amount of intervening tissue. Hence the mechanicore- 
ceptors are not necessarily exposed directly to what is 
ordinarily called the stimulus as chemoreceptors are, but 
they may be excited more or less indirectly. Our organs 
of touch and of hearing, therefore, may be lodged in the 
deeper part of the skin or the head without interfering 
in any serious way with their efficiency. All chemorecep- 
tors on the other hand are necessarily either upon the 
exposed surfaces of the body or are provided with pores 
that lead from these surfaces directly to the receptors 
themselves. This condition is in a way merely a corollary 
of what has already been stated about chemical stimula- 
tion, for if the organs of smell, taste and the like are 
acted on chemically by their appropriate stimuli, these 
stimuli must of necessity come into direct contact with 
the given terminals. 

2. Differences among Chemoreceptors. The chemore- 
ceptors agree then in the general character of their stim- 
uli. Such stimuli are certain chemically active materials 
in solution applied directly to the receptors themselves. 
The variety that these organs exhibit ought, therefore, 
to turn more or less on the extent of their differentiation 
in relation to the chemical diversity of the environment. 
The degree of this organic differentiation, however, has 
been very inadequately worked out. Almost nothing is 



170 SMELL, TASTE, ALLIED SENSES 

known of the stimuli for the vomero-nasal organ, and 
very little has been done on those for the common chemi- 
cal sense. The senses of smell and of taste are naturally 
much better known. When their stimuli are compared 
they are found in general to belong to different categories 
of material; what is smelled is generally not tasted and 
what is tasted is not smelled. 

These two categories of substances afford an impor- 
tant basis for comparing taste and smell. This can be done 
from the standpoint of the minimum concentrations of 
materials that serve as stimuli for the two sets of recep- 
tors. Bitter substances are apparently the most effec- 
tive stimuli for the sense of taste. Quinine hydrochloridc 
can be tasted in a solution as weak as 0.00004 molar, 
but this threshold is exceeded by that of what is probably 
the most bitter of all substances strychnine. According 
to Gley and Richet (1885) the weakest solution in which 
the bitter taste of strychnine hydrochloride can be distin- 
guished contains only 0.0004 gram of this substance in 
one liter of water. This is approximately equivalent to 
one and a half million ths of a molar solution (1.48xlO~ 6 
molar), and much exceeds in this respect the efficiency 
of quinine. One of the strongest odors known is that of 
mercaptan of which according to Fischer and Penzoldt 
(1886), 0.01 milligram evaporated in 230 cubic meters of 
air gives a perceptible smell. Assuming the substance 
used by these investigators to have been methyl mercap- 
tan, such a dilution would bo represented by about a 
million-millionths molar solution (9xlO Ki ) or approxi- 
mately one and a half million times more dilute than the 
weakest solution of strychnine that can be tasted. Thus 
the olfactory receptor is open to stimulation by a very 



INTERRELATION OF THE CHEMICAL SENSES 171 

much weaker concentration than the gustatory one is. 
It might be maintained, however, that the line of ar- 
gument used in the last paragraph is invalid because it 
is based upon measurements of one substance for taste 
and another for smell, and that, therefore, the two sets 
of figures are not fairly comparable. But the conclusion 
just reached is also supported by determinations made 
with a single substance. Ethyl alcohol is soluble in both 
water and oil and is one of the relatively few substances 
that has at once both taste and smell. As a matter of 
fact it is also a stimulus for the common chemical sense. 
Hence it may be conveniently employed for comparing all 
three classes of receptors. When such a test is made, 
it is found that the weakest concentration of alcohol vapor 
that can be smelled is about 0.000125 molar and that the 
weakest aqueous solution of this substance that can be 
tasted is 3 molar. To stimulate the common chemical 
sense with ethyl alcohol requires an aqueous solution of 
strength 5 to 10 molar. Hence so far as ethyl alcohol 
is concerned smell may be said to be about 24,000 times 
more delicate than taste and about 60,000 times more 
delicate than the common chemical sense. From the 
standpoint of a single substance then, smell must be ad- 
mitted to be vastly more efficient than either taste or 
the common chemical sense both of which lie in this re- 
spect close together (Parker and Stabler, 1913). Unfor- 
tunately the stimulation of the vomero-nasal organ has 
not yet been studied so that its capability from this 
standpoint is not known, but, judged from its structure, it 
probably has a receptive efficiency not far from that of 
the olfactory organ. In that case the chemoreceptors of 
vertebrates would fall into two groups, the olfactory and 



SMELL, TASTE, ALLIED SENSES 

voniero-nasal organs with high efficiency and the common 
chemical receptors and organs of taste with relatively 
low efficiency. These two sets of organs might in this 
respect be compared with scales, the organs of taste and 
of the common chemical sense resembling ordinary scales 
on which only gross amounts are weighed and the organs 
of smell and the vomero-nasal organs resembling chemi- 
cal balances on \vhich small weights may be determined. 

As olfaction deals effectively with very minute 
amounts of substance and gustation only with much 
greater amounts, it follows that materials that have be- 
come highly attenuated by being broadly spread from 
their sources either in water or in air may nevertheless 
still be concentrated enough to stimulate the organs of 
smell though they can have no possible effect upon those 
of taste. Such faint odors are the means whereby ani- 
mals scent their food, find their mates, or avoid their 
enemies. Hence the olfactory organ has been appropri- 
ately classed as a distance receptor or exteroceptor, to 
use a convenient term from Sherrington (1906), in that 
the impulses to which it gives rise commonly direct the 
animal toward distant points or away from them. 

Taste and, in the higher vertebrates at least, the com- 
mon chemical sense are stimulated only by relatively con- 
centrated solutions such as occur in connection with the 
food. Hence the responses that these organs call forth 
are concerned with the swallowing of food, with the re- 
jection of material taken into the mouth, with mastication 
and saliva and the like. These receptors are, therefore, 
rightly classed as interoceptors though it must be re- 
membered, as Herrick (1918) has pointed out, that in 
some fishes, such as the catfishes, taste-buds serve in the 



INTERRELATION OF THE CHEMICAL SENSES 173 

discovery of food as well as in its appropriation, and 
partake, therefore, more or less of the nature of extero- 
ceptors. Although olfaction has a function independent 
and separate from that of gustation in scenting mates 
or enemies and gustation has a function independent of 
olfaction initiates the feeding reflexes both muscular and 
noxious material, both senses are intimately associated 
in feeding. Food is found and the digestive secretions 
are started through smell; it is swallowed and these se- 
cretions are intensified ordinarily through taste. Thus 
olfaction initiates the feeding reflexes both muscular and 
secretory and gustation reinforces and completes them. 
It is remarkable that in some fishes like the catfishes 
(Amiurus) and especially the dogfishes (Mustelus; 
Parker, 1914) feeding scarcely ever occurs, even when the 
fishes are starving and food is present, unless the process 
is initiated through olfactory reflexes. These seem to 
be essential for that chain of events that result in the 
final swallowing of the food, a condition that shows how 
intimately smell and taste are interwoven in the verte- 
brate organization. 

Smell and taste, though thus most closely involved in 
the feeding reflexes, are nevertheless perfectly distinct. 
As long ago as 1821 Cloquet (Larguier des Bancels, 1912) 
showed that on closing the nose by pinching the nostrils 
smell can be eliminated and only taste remains. Under 
such circumstances it is surprising to those who have 
not previously tried the experiment to discover how small 
a proportion of our food sensations are due to taste and 
how large a one to smell. A cold in the head commonly 
eliminates smell and leaves taste. It reduces a person 
to a state in which food is often described as without 



174 SMELL, TASTE, ALLIED SENSES 

flavor, for only sour, saline, sweet, and bitter tastes can 
be sensed and onion produces the same sweetish taste 
that apple does. The separateness of smell and taste 
depends doubtless upon the conditions already described. 
Smell is excited in general by one set of substances ; taste 
by another. Smell calls for only very weak solutions; 
taste requires relatively strong ones. It may also be that 
these two senses differ in the nature of the solutions that 
activate them; taste is attuned to substances that form 
aqueous solutions, smell to those that dissolve in oil. 
Cell surfaces are commonly believed to be diphasic in 
that they are composed of a mixture of two materials one 
oily and the other aqueous. The gustatory hairs may be 
so constituted that the aqueous constituent is the avenue 
of entrance for the stimulating substance and the olfac- 
tory hairs so that the oily one is the inlet. If such is the 
case, this feature may also be an important difference 
between smell and taste. 

3. Groups of Chemical Receptors. Taste and smell 
are two of the five senses ordinarily attributed to man. 
But in the detailed study of the human senses not one 
has escaped a kind of functional subdivision whereby it 
has been shown to be more than a single sense. Thus the 
internal ear originally regarded by physiologists as 
purely an organ of hearing, was shown by Flourens in 
1828 to be concerned in a most important way with bodily 
equilibrium. From this standpoint the ear takes on the 
character of a double sense organ. This duplicity is 
especially well marked in certain fishes in which the 
membranous labyrinth is completely divided in two cor- 
responding to the functional differentiation already in- 
dicated; one of these parts consists of the utriculus with 



INTERRELATION OF THE CHEMICAL SENSES 175 

its three semicircular canals and has to do with equi- 
librium and the other of the sacculus and its appended 
lagena and is concerned with hearing. Even so unified 
an organ as the human eye is made up of an intermingling 
of two receptive fields, for, as originally suggested by 
Schultze (1866) and as elaborated by von Kries (1904), 
the retinal rods are concerned with colorless vision in 
dim light and the cones with color vision in bright light. 
Thus the eye is differentiated for two kinds of sight, one 
by night and the other by day. The integumentary sense 
originally supposed to be unitary, was shown by Blix in 
1884 to consist of at least three senses, cold, warm, and 
pressure. To these were added in 1896 by von Frey a 
fourth, pain. Thus it is clear that the conception of five 
senses for man is wholly inadequate and though numbers 
are perhaps not the best way of indicating the sensory 
equipment of human beings or in fact of any other ani- 
mal, it is not without interest to record the opinion of 
Herrick (1918) that the classes of human receptors are 
now known to be more than twenty. 

The chemoreceptors, represented in the older accounts 
by the organs of taste and smell, have no more escaped 
this process of increase than have the other sense organs. 
The vomero-nasal organ appears to be a kind of accessory 
receptor for smell and the common chemical sense is ap- 
parently a primitive form of gustatory organ. But in 
addition to these subsidiary receptors, the true olfactory 
surfaces as well as the gustatory areas are not homo- 
geneous, but are marked by local receptive differentiation. 
This is especially well illustrated by the so-called sense 
of taste. This, as has already been shown in the preceding 
chapter, is in reality not a single sense, but, in accordance 



176 SMELL, TASTE, ALLIED SENSES 

with Oehrwall's opinion (1901), must be regarded as 
generic and to consist of at least three and probably four 
senses, namely the sense of sour, of saline, of bitter, and 
of sweet. These senses are really distinct and separate. 
They have independent receptors and give rise to sensa- 
tions that do not intergrade. Their association under 
one head as members of the sense of taste is in a way a 
misconception due doubtless to the fact that in ordinary 
activity all four senses are commonly in operation at 
once, and hence acquire a certain degree of functional as- 
sociation. Taste then is not the name for a single sense 
but for a group of senses and it is likely that smell is of 
the same nature, but until olfaction is better understood, 
it is impossible to indicate the elements of which it is com- 
posed. Thus the chemical senses, like the others already 
briefly enumerated, show the same tendency to increase 
in number as they become better known. 

4. Classification of Receptors in General. A detailed 
investigation of the chemoreceptors leads to an increas- 
ing multiplicity of elements as in the other receptor sys- 
tems, and raises the question of what constitutes a unitary 
sense and how such units are related. When one or 
more similarly organized receptors are excited to activity 
by a single category of stimuli and give rise to the same 
kind of sensation we think of the aggregate as a sense. 
Thus when a deforming pressure impinges upon any part 
of the skin, touch receptors are stimulated and we re- 
ceive a uniform impression characteristic of the sense of 
touch. Or when one of a variety of sounds falls upon 
the ear, we experience hearing. In the second instance 
the stimulus, different sounds, is open to much greater 
variety than in the first where the stimulus is, a deform- 



INTERRELATION OF THE CHEMICAL SENSES 177 

ing pressure, and in a corresponding way the sensations 
in hearing are much more diverse than those in touch. 
But it is still reasonable to regard hearing as one sense, 
for its various stimuli grade into one another as its sen- 
sations do. With taste on the other hand such is not the 
case. The acid stimulus as an external agent is entirely 
distinct from the stimuli for the other tastes and the 
sour sensation as an internal state does not grade into 
other gustatory sensations. This separateness in stim- 
uli and in sensations is characteristic of the four kinds 
of tastes and justifies their acceptance as separate senses, 
a division that is not permissible in hearing. To con- 
stitute a single sense implies a reasonable similarity in 
stimulus, receptive mechanism, and sensation. 

But, as previously pointed out, the initiation of sen- 
sations is a function of only a limited number of the 
human receptors. Many of these organs are concerned 
with activities entirely unassociated with sensation; 
hence to speak of them as representing a sense seems 
somewhat inconsistent. If the term receptor is an im- 
provement over that of sense organ because of its free- 
dom from implications concerning sensation, it might be 
well for the same reason to substitute some other term 
for sense, such, for instance, as recept. 1 In that case a 
recept is that aggregate of action that occurs where the 
receptive arm of any reflex arc goes into normal activity 
irrespective of whether this activity is productive of a 
sensation or not. The recept then includes all the oper- 

1 1 am fully aware that this term has already been appropriated by 
the psychologists for a very different purpose, but as they have taken 
almost all the terms in the language for their own use, I do not hesitate 
to reappropriate this one to fill the present need. 

12 



178 SMELL, TASTE, ALLIED SENSES 

ations from the reception of the stimulus to those central 
changes that mark the entrance of the impulse into the 
central organ including the production of a sensation, 
if such occurs. 

Where a recept is concerned with sensation, the pro- 
duction of this state may be regarded as its final step. A 
sensation, then, is an activity in a particular region or 
spot of the central nervous organ marking the central 
end of the receptive portion in a reflex arc. Experience 
has shown that, irrespective of the means by which this 
central region is stimulated, it calls forth only one kind 
of sensation. This in a way is a restatement of the mod- 
ern view of Miiller's specific energy of the nerves, for, 
according to this principle, however a particular sense 
organ, or conducting trunk, or nerve center may be stim- 
ulated, only one kind of sensation results. In other 
words the character of a sensation is not determined by 
peripheral organs but is strictly a central affair and sen- 
sations are different not because of the different sources 
of the incoming impulses, but because of the different 
central spots excited. Since the anatomical connections 
are such that a particular receptor always leads to a 
special central region, it follows that such a receptor be- 
comes thus associated with a given sensation. Hence 
where sensations occur they may be used in distinguish- 
ing receptors, but in the many recepts that are unassoci- 
ated with sensation this feature naturally cannot be called 
upon as a means of discrimination. 

Although numerous receptors are in no way concerned 
with sensations, there are no receptors that are not ac- 
tuated by stimuli. Hence the stimulus affords a more 
general basis for discriminating between receptors than 



INTERRELATION OF THE CHEMICAL SENSES 179 

the sensation does. The two groups of chemoreceptors 
and of mechanicoreceptors, already frequently alluded 
to, show how fundamental this method of classification 
is, for these two groups represent the two well-recognized 
activities of our material surroundings and together may 
be put in strong contrast with radioreceptors such as the 
organs for heat and for cold and the eye, all of which are 
stimulated by radiant energy. 

These three classes constitute the fundamental groups 
of receptors and under some one of these heads every 
such organ should find its place. To the chemoreceptors 
discussed in this volume may possibly be added those 
on the wall of the stomach that according to Carlson 
(1916) have to do with appetite. The receptors for pain 
are possibly stimulated by the chemical action of ab- 
normal tissue juices and the endings for thirst may also 
depend upon some such form of activation (Cannon, 
1918), though both of these organs may belong to the 
group of the mechanicoreceptors (Muller, 1920). To the 
mechanicoreceptors belong; unquestionably those termi- 
nals that are excited by a deforming pressure such as 
the receptors for touch, for pressure, including the or- 
gans for equilibrium, and for hearing. Very probably 
pressure is the stimulus for muscle, tendon, and joint re- 
ceptivity and the sense of fullness in cavities. Pressure 
produced by the contraction of the muscular walls of 
the stomach appears to be the stimulus for the hunger 
pang (Cannon and Washburn, 1912). The lateral-line 
organs of fishes and amphibians give every evidence of 
being* mechanicoreceptors. Finally radiorecepto-rs are 
those organs that are stimulated by radiant energy such 
as the heat organs, the cold organs, and the eye. 



180 SMELL, TASTE, ALLIED SENSES 

To ascertain into which of these three groups a re- 
ceptor falls it is necessary to know how it is stimulated 
after which its classification is simple and immediate. 

Although a grouping of receptors based upon their 
stimuli will of necessity always be complete, this plan of 
arrangement is not entirely devoid of difficulties. Chief 
among these is the fact that the same stimulus may ac- 
tivate what we know from other standpoints to be differ- 
ent receptors. Thus, as already stated, parabrombenzoic 
sulphinide excites sweet receptors as well as bitter ones, 
and strong material vibrations will stimulate the organs 
of touch as well as the ear. But such instances appar- 
ently occur only between closely related receptors, for 
the organs for sweet and for bitter are so closely related 
as to be regarded by many as belonging to one category 
and hearing is certainly very near akin to touch. Herrick 
(1918) has discussed the definition and classification 
of receptors and has urged for this purpose the use of 
four criteria : the sensation, the stimulus, the sensory 
mechanism, and the type of response. In his opinion, 
however, none of these affords a wholly satisfactory basis 
for discrimination and grouping, operations that can be 
successfully carried out only when sufficient information 
is at hand. But experience scarcely warrants such a 
conclusion, for it is much more difficult now to discover 
the interrelation of the twenty or more human receptors 
with all that is known about them than it was to make a 
corresponding statement about the original five. The real 
difficulty lies in the fact that the numerous receptors that 
we now recognize have undergone varying degrees of 
differentiation and hence their mutual affinities are ex- 
tremely diverse. This brings us at once face to face with 



INTERRELATION OF THE CHEMICAL SENSES 181 

one of the problems of this inquiry, namely, the genetic 
relations of receptors. 

5. Genetic Relations of Chemoreceptors. The three 
sets of receptors mentioned in the last section, the 
chemoreceptors, the mechanicoreceptors, and the radiore- 
ceptors, are more than mere convenient assemblages ; they 
represent natural groups of organs whose relations with- 
in each group have a certain genetic character. This can 
be illustrated by the chemoreceptors. 








Fia. 37. Diagrams illustrating the receptor systems of the following verte- 
brate chemoreceptors: a, olfactory organ and vomero-nasal organ; b, organ of the 
common chemical sense; c, gustatory organ. After Parker, 1912. 

If the structure of the several vertebrate chemore- 
ceptors is compared, it will be found that they present 
three types of organization (Fig.37). These types can 
be best appreciated from the standpoint of their constit- 
uent neurones. In the olfactory and vomero-nasal 
organs the neurones have cell bodies in the receptive 
epithelium and their axons extend as nerve-fibers from 
these bodies into the central organ. In the common chem- 
ical organs the receptors are free-nerve terminations in 
the mucous epithelium of the mouth, the nose, the eye and 
other such apertures, from which axons provided with 



182 SMELL, TASTE, ALLIED SENSES 

deep-seated cell-bodies extend into the central organs. 
Finally, in the gustatory organs the taste-buds are com- 
posed of receptive epithelial cells that are in synaptic 
relations with nerve terminals essentially like free end- 
ings from which axons with deep-seated cell-bodies pass 
into the central organs. These three types of structure 
include, so far as is known, all the vertebrate chemorecep- 
tors. To a common stimulus, like ethyl alcohol, the ol- 
factory type has been shown to have by far the lowest 
threshold followed in order by the gustatory and the com- 
mon chemical types both of which are near together in 
this respect. 

When these three types are compared with the recep- 
tors of other animals, it is seen that the olfactory type 
reproduces almost exactly that found in the skins of many 
invertebrates, and that the other two types are character- 
istically vertebrates. The integument of animals even as 
simply organized as sea-anemones is rich in receptive cells 
that reproduce in almost eveiy detail the conditions of 
the vertebrate olfactory neurones. Not only do these 
lowly organized forms show this structural similarity in 
their integumentary cells, but they are known to be so 
responsive to minute amounts of material wafted from 
distant food through the water to them that they have 
been for a long time past credited with olf action (Pollock, 
1883). Thus the vertebrates olfactory epithelium and the 
integument of aquatic invertebrates are strikingly alike. 
It is more than probable that the vertebrates have 
descended from ancestors whose skin was an epithelium 
like that on the exterior of a sea-anemone and that, as 
this skin thickened over most of the body to give the 
necessary protection to the slowly metamorphosing ani- 



INTERRELATION OF THE CHEMICAL SENSES 183 

mal, the future olfactory region remained unchanged and 
thus retained its original invertebrate character. This 
region became the olfactory epithelium of the developing 
vertebrate, the most primitive chemoreceptor in this 
group of animals. 

The organs next in this series were the common chem- 
ical receptors. The neurones for these organs were 
differentiated from the neurones of the primitive inverte- 
brate skin by a central migration of their cell-bodies till 
they became part of the spinal ganglia and thus left in the 
integument free-nerve terminations as receptors. This 
type of chemoreceptor is found generally in the skin of 
fishes and amphibians and in the mouths, nasal chambers 
and other moist cavities of the air-inhabiting vertebrates. 

The third and last type of the vertebrate chemorecep- 
tor is the gustatory organ. In this type the conducting 
neurone presents exactly the condition met with in the 
common chemical receptor excepting that its nerve ter- 
minals, instead of being free in the integument, are asso- 
ciated with epithelial taste-buds. This type of receptor 
was probably derived from the second type by the appro- 
priation of taste-cells from the integumentary epithelium. 
Thus the three types of vertebrate chemoreceptors 
appear to be genetically related in that the olfactory 
organs represent what may be called the first generation, 
the common chemical the second, and the gustatory the 
third (Parker, 1912). 

But within each type much detailed differentiation 
has taken place. It seems to be quite impossible to ex- 
plain the variety of olfactory sensations without assum- 
ing a differentiation among the receptors of the olfactory 
field. In the common chemical sense the receptors on 



184 SMELL, TASTE, ALLIED SENSES 

the moist surfaces of the eye, judged by the sensations 
they give rise to, are distinguishable from those in the 
epithelium of the mouth and of the nose. But this special 
differentiation is best seen in the gustatory organs. Here 
three and probably four well defined senses can be dis- 
tinguished, namely, sour, saline, sweet, and bitter. And 
though separate receptors for these four senses have not 
as yet been distinguished structurally, their functional 
separation is beyond doubt. 

It is because of the repeated differentiations that 
characterize the evolution not only of the chemoreceptors 
but of the other groups of like organs that a classification 
of them or even a simple enumeration proves to be so 
unsatisfactory. For they are not unitary elements that 
can be counted like the fingers on the hand nor are they 
sufficiently co-ordinated to make classifications easy and 
natural. They are like the whole organism itself in that 
they exibit that kind of diversity that characterizes evo- 
lutionary flux. 

6. BIBLIOGRAPHY 

BLIX, M. 1884-1885. Exporimentclle Beitriige zur Losung der Frage 

iiber die specifische Energie der Hantnerven. Zcitschr. Biol., Bd. 20, 

pp. 141-156, Bd. 21, pp. 145-160. 
CANNON, W. B. 1918. The Physiological Basis of Thirst. Proc. Roy. 

8oc., London, B, vol. 90, pp. 283-301. 
CANNON, W. B., AND A. L. WASHBUBN. 1912. An Explanation of Hunger. 

Antrr. Join 4 . Physiol., vol 29, pp. 441-454. 
CARLSON, A. J. 1916. The Control of Hunger in Health and Disease. 

Chicago, 319 pp. 
FISCHER, E., UND F. PENZOLDT. 1886. Ueber die Empfmdlichkeit dee Geruch- 

sinnes. ftitzb. phys.-med. Soc., Erlangen., Heft 18, pp. 7-10. 
FLOURENS, M. P. 1828. Experiences sur les canaiix semi-circuluires de 

1'oreillo chez les oiseaux. Ann. Sci. Nat., tome 15, pp. 113-124. 



INTERRELATION OF THE CHEMICAL SENSES 185 

VON FREY, M. 1896. Untersuchungen iiber die Sinnesfunotionen der 

menschlichen Haut. A bh. Sachs. Gesell. Wissensch., math.-phys., Cl., 

Leipzig, Bd. 23, pp. 169-266. 
GLEY, E., ET C. RICHET. 1885. De la sensibilite gustative pour les alcaloides. 

Compt. rend. Soc,. Biol., Paris, tome 37, pp. 237-239. 
HERRICK, C. J. 1918. An Introduction to Neurology. Philadelphia and 

London, 394 pp. 
VON KRIES, J. 1904. Die Gesichtsempfindungen. Nagel, Handb. Physiol. 

Menschen, Bd. 3, pp. 109-282. 

LARGUIER DES BANCELS, J. 1912. Le Gout et 1'Odorat, Paris, 94 pp. 
MULLER, L. R. 1920. Ueber den Durst und ueber die Durstempfindung. 

Deutsch. med. Wochenschr., Bd. 46, pp. 113-116. 
OEHRWALL, H. 1901. Die Modalitals- und Qualitatsbegriffe im der; Sinnes- 

physiologie und deren Bedeutung. Skandinavisches Archiv. Physiol., 

Bd. 11, pp. 245-272. 

PARKER, G. H. 1912. The Relation of Smell, Taste, and the Common Chem- 
ical Sense in Vertebrates. Jour. Acad. Nat. Sci., Philadelphia, vol. 

15, pp. 221-234. 
PARKER, G. H., 1914. The Directive Influence of the Sense of Smell in the 

Dogfish. Bull. United States Bur. Fish., vol. 33, pp. 63-68. 
PARKER, G. H., AND E. M. STABLER. 1913. On Certain Distinctions between 

Taste and Smell. Amer. Jour. Physiol., vol. 32, pp. 230-240. 
POLLOCK, W. H. 1883. On Indications of a Sense of Smell in Actiniae. 

Jour. Linn. Soc., Zool., vol. 16, pp. 474-476 

RIBOT, T. 1920. Le Gout et 1'Odorat. Jour. Psych., ann. 17, pp. 5-15. 
SCHULTZE, M. 1866. Zur Anatomi und Physiologie der Retina. Arch. 

mik. Anat., Bd. 2, pp. 165-286. 
SHERRINGTON, C. S. 1906. Thd Integrative Action of the Nervous System. 

New York, 411 pp. 






' 
2 

/. . 

...; 



INDEX 



Acetic acid, 137 

Activators, 18, 21 

Adequate olfactory stimuli, 80 

Aducco, V., et U. Mosso, 153, 156, 

162 

Ageusia, 156 
Alcock, N., 33, 39 
Alcohol, 171 
Alkaline taste, 134, 135 
Alkaloids, 141 
Allison, V. C., and S. H. Katz, 52, 

55, 56, 88 
Althaus, J., 81, 88 
Amiurus, 116, 160, 173 
Ammocoetes, 104 
Amphioxus. 103 
Anosmia, 71, 72, 83 
Von Anrep, B., 153, 162 
Anton, W., 92, 100 
Appetite, 179 

Arnstein, C., 29, 39, 121, 122, 128 
Aronsohn, E., 57, 58, 60, 63, 70, 81, 

83, 85, 88 
Asai, T., 33, 34, 39 
Auxogluc, 144 

Babuchin, A., 29, 39 

Backman, E. L., 60, 61, 77, 88 

Baginsky, B., 125, 128 

Baglioni, S., 64, 68, 88 

Ballowitz, E., 31, 39 

Barbus, 64 

Barral, F., et A. Ranc, 162 

Basal cells, 29, 30, 119 

Beaunis, H., 155, 162 

Becker, C. T., und R. O. Hertzog, 

138, 162 

Bedford, E. A., 39 
Beer, T., A. Bethe, und J. von 

Uexkiill, 21 
Benzoin, 70 



Bethe, A., 17, 21, 22 

Bidder, F., 48, 87 

Bishop, J., 43, 88 

Bitter taste, 140, 150 

Blakeslee, A. F., 72, 88 

Blaue, J., 34, 39 

Blix, M., 175, 184 

Bowden, H. H., 39 

Braeuning, H., 108 

Broman, I., 99, 100 

Brookover, C., 98, 100 

Von Brunn, A., 28, 29, 31, 34, 35, 39, 

96, 97, 98, 100 
Bulbar fibers, 123 
Bunzel, R., 147, 164 

Cannon, W. B., 179, 184 

Cannon, W. B., and A. L. Washburn, 
179, 184 

Carlson, A. J., 179, 184 

Castronovo, A., 29, 33, 34, 40 

Catfish, 65 

Chemical relations of odors, 76 

Chemoreceptors, 169, 175, 176, 179 

Chorda tympani, 126 

Chumming, 68 

Circus movements, 67 

Classification of receptors, 176 

Cloquet, 173 

Cocaine, 153 

Coelenterates, 19 

Coghill, G. E., 106, 108 

Cohn, G., 78, 88, 141, 144, 162 

Cold organs, 179 

Cole, L. W., 103, 107, 108 

Common chemical organs, 181 

Common chemical sense, 102 

Comparative distribution of taste- 
buds, 115 

Comparative physiology of taste, 160 

Component theory of taste, 152, 158 

187 



188 INDEX 

Concha?, 23, 27, 38 Gawrilenko, A., 93, 100 

Copeland, M., 66, 68, 88 Gemmal fibers, 123 

Crozier, W. J., 103, 104, 107, 108, Genetic relations of chemoreceptors, 

138, 139, 158, 162, 163 181 

Gushing, H., 125, 126, 128 Gertz, H., 147, 163 

Glaser, 0., 71, 89 

Diemyctylus, 68 Gley, E., 162 

Disse, J., 34, 36, 39, 40 Gley, E., et C. Richet, 141, 163, 170, 

Distribution of taste, 148 185 

Dogfish, 64, 173 Glucophore, 144 

Dogiel, A. S., 33, 40, 115, 123, 128 Goldfish, 63 

Drasch, 0., 125, 128 Golgi, 32 

Drugs and taste, 152 Goldscheider, A., und H. Schmidt, 

Duges, A., 43, 88 151, 163 

Durand, A., 48, 60, 68, 88 Graber, V., 15, 22 

Durrans, T. H., 88 Graberg, J., 110, 114, 116, 118, 120, 

Duval, 134, 163 12 8, 129 

Grassi, V., und A. Castronovo, 29, 

Von Ebner, V., 116, 128 33> 34; 40 

Ecker, A., 28, 40 Greenberg, D., 69, 91 

Eckhard, C., 28, 31, 40 Group of chemical receptors, 174 

Edgeworth, 152 Gustatory chiasma, 125 

Ehrlich, P., 29, 40 Gustatory contrasts, 156 

Electrical stimulation of taste, 147 Gustatory nerves, 123 

Eschricht, D. F., 43, 89 Gustatory nerve fibers, 127 

Ethmoid cells, 26 Gustatory organs, 110, 182 

Extrabulbar cells, 120 Gustatory senses, 158 

E y e 179 Gustatory stimuli, 133 

Gymnema, 152 

Fischer, E., und F. Penzoldt, 53, 54, Gvmnemic acid, 153 

56, 89, 184 

Flourens, M. P., 174, 184 Hahn, R., 132, 164 

Foliate papillae, 113 Haller, B., 73, 115, 129 

Fontana, A., 153, 163 Hamlin, H. E., 100 

Franke, G., 45, 47, 89 Hammerhead shark, 67 

Free-nerve endings in olfactory re- Hiinig, D. P., 132, 149, 163 

gion, 34 Harvey, R. B., 138, 163 

Frentzel, J., 156, 163 Haycraft, J. B., 57, 77, 87, 157, 162 

Von Frey, M., 54, 89, 135, 163, 175, Hearing, 176 

185 Heat organs, 179 

Frolich, R., 44, 86, 89 Heidenhain, M., 113, 115, 116, 117, 

Frontal sinus, 26 119, 129 

Fungiform papilhe, 112, 150 Heiderich, F., Ill, 114, 129 

Fusari, R., et A. Panasci, 121, 128 Henle, J., 136, 163 

Kenning, H., 60, 68, 69, 74, 75, 76, 

Garman, S., 89 78, 79, 80, 82, 87, 99, 100, 158, 163 

Gaupp, E., 100 Henry, C., 163 



INDEX 189 

5 el VT'i L '' I 41 '/ 55 ' 163 Kahlenberg, L., 135, 136, 137, 138, 

Herhtzka, A, 135, 140, 142, 153, 139, 140? 164 

154, 163 Kallius, E., 31, 34, 40, 100, 116, 129 

Hermann, F., Ill, 118, 119, 120, 129 Kamon, K., 34 40 

Herrick C. J., 106, 108, 115, 124, Karpman, B., 49, 91 

129, 160, 161, 163, 172, 175, 180, Kastle, j! H., 154, 164 

Hertzog, R. 0, 138, 162 g^ ** ,f ,f 56 > 88 . 

Heymans, a, 156, 163 ^ ?, 136, 139, 140, 

Hisiology of vomero-nasal organ, 96 3' } ^ 153 ' 155 ' 157 ' 158 > 

' 135 ' 139 ' Kieso^ F, und R. Hahn, 164 



A 11, 129 
Hofmann, F, und'l, Bunzel, 147, ^^ 153,? 64 



Honigschmied, J., 124, 131, 155 

Hooper, D, 153, 164 Krause, W., 28, 40, 114, 126, 129 

Howell, W. H, and J. H. Kastle, Kremer > J- H., 62, 89, 157, 164 

154, 164 Von Kries, J., 175, 185 
Humboldt, 146 

Hunger, 179 Landacre, F. L., 128, 129 

Huyer, C., 78, 89 Larguier des Bancels, J., 61, 87, 162, 

173, 185 

Inadequate gustatory stimuli, 145 Larsell, 0., 100 

Inadequate olfactory stimuli., 80 Latency of taste, 154 

Innervation of taste-buds, 120 Lateral-line organs, 15, 179 

Insipidity, 135 Von Lenhossek, M., 34, 40, 97, 98, 
Inspiration, 48 100, 118, 120, 121, 123, 129, 130 

Integumentary sense, 175 Leydig, F., 16, 115, 117, 130 

Intermediate zone, 32 Lingual nerve, 125 

Interrelation of the chemical senses, Linnaeus, 73 

167 Location of taste, 132 

Intrabulbar fibers, 122 Loeb > J -> 62 > 89, 108 

Intragemmal fibers, 122 Loeb ' R - F -> 62 > 89 

Intragemmal spaces, 120 Loven, C., 110, 117, 130 

Irritants, 44 Lubbock, J., 15, 22 

Lucas, K., 13 

Jacques, P., 121, 129 Luciani, L., 68, 87, 158, 162 
Jacobson cartilage, 98 

Jagodowski, K. P., 32, 33, 34, 40, McCotter, R. E., 94, 101 

62, 89 Magendie, F., 43, 89 

Johnston, J. B., 115, 129 Marchand, L., 162 

Jourdan, E., 15, 22 Maxillary sinus, 26 



190 



INDEX 



Mechanicoreceptors, 179 
Merkel, P., 118, 130 
Metallic taste, 134 
Meyer, S., 125, 127, 130 
Von Mihalkovics, V., 99, 101 
Minimum olfactory stimulus, 49 
.Merrill, A. D., 33, 34, 40 
Mosso, U., 153, 156, 162 
Miiller, Johannes, 68, 152, 178 
M filler, L. R., 179, 185 
Munch, F., 115, 130 
Myers, R. G., 144, 145, 164 

Xagel, W., 48, 63, 64, 67, 76, 87, 
89, 103, 109, 156, 158, 162, 164 

Nasal cavities, 23 

Nasal membranes, 26 

Nasolacrimal duct, 26 

Nerves of ol faction, 42 

Nerve terminals of common chem- 
ical sense, 104 

Nettesheim, K., 143, 165 

Neurones, 181 

Neutralizing odors, 85 

Newt, 68 

Odiometer, 52 

Odor mixtures, 83 

Oehrwall, H., 135, 150, 151, 157, 

158, 160, 164, 176, 185 
Oertly, E., and R. G. Myers, 144, 

145, 164 

Olfaction and radiation, 62 
Olfaction in fishes, 63 
01 faction and solvents, 60 
Olfactometer, 50 
Olfactory acuity, 53, 77 
Olfactory cell, 30 
Olfactory cleft, 25 
Olfactory epithelium, 27, 28 
Olfactory fatigue, 69 
Olfactory flagella, 32 
Olfactory hairs, 31 
Olfactory nerve, 35, 42 
Olfactory nerve fibers, 29 
Olfactory organ, 23, 36, 181 
Olfactory organ of fishes, 37 



Olfactory potency, 77 

Olfactory prism, 75 

Olfactory reflexes, 86 

Olfactory sense buds, 34 

Olfactory stimulus, 57 

Olfactory vesicle, 31 

Olmsted, J. M. D., 68, 89, 127, 130 

Organ of Jacobson, 92 

Osmophoric groups, 78 

Pain, 179 

Panasci, A., 121, 128 

Papillae of tongue, 111 

Parabrombenzoic sulphinide, 154 

Parker, G. H., 22, 66, 89, 90, 103, 

104, 106, 109, 161, 164, 173 183, 

185 
Parker, G. H., and E. M. Stabler, 

55, 141, 143, 165, 171 
Parker, G. H., and A. P. Van Heu- 

sen, 148, 165 
Passage of air through nasal cavity, 

44 

Passy, J., 54, 55, 77, 90 
Paulsen, E., 45, 46, 47, 48, 90 
Pawlow, J. P., 86 
Penzoldt, P., 53, 54, 56, 89 
Peribulbar fibers, 122 
Perigemmal fibers, 122 
Peter, K., 40 

Physiology of gustation, 132 
Physiology of ol faction, 42 
Picht, P., 43, 90 
Pier cells, 118 
Tiutti, A., 133, 165 
Pollock, W. H., 182, 185 
Polymorphic cells, 33 
Ponzo, M., Ill, 130, 165 
Potassium chlorate, 156 
Prins, H. J., 90 

Qualities of odors, 72 
Qualities of tastes, 134 

Radioreceptors, 179 
Ramon y Cajal, S., 29, 34, 40, 97, 
98, 101, 



INDEX 



191 



Ranvier, L., 118, 120, 125, 127, 130 
Read, E. A., 40, 97, 98, 101, 
Recept, 177 
Receptors, 18 
Reflex action, 16 



Retzius, G., 29, 31, 34, 40, 41, 97, g 143 

"I/~\T i i f\ i n i 1 rr* i c\ o io/\ O ' 



Sternberg, W., 87, 134, 154, 162, 165 

Stich, A., 134, 165 

Stimulation of chemoreceptors, 167 

Stovaine, 153 

Substances with two tastes, 153 



101, 119, 121, 122, 123, 130 
Reuter, C., 72, 90 
Ribot, T., 185 

Richards, T. W., 136, 137, 165 
Richet, C., 141, 163, 170 
Risser, J., 68, 90 
Ritter, 147 
Rod cells, 118 
Rollett, A., 72, 90 
Rosenthal, J., 146, 147, 165 
Rubaschkin, W., 35, 41 

Saccharine, 142 
Saline taste, 139, 150 
Sandmeyer, W., 125, 127, 130 



Sulzer, 145 
Supporting cells, 117 
Sustentacular cells, 29, 30 
Sweet taste, 142, 150 
Symington, J., 101 
Systems of odors, 73 

Tadpole, 68 

Taste alteration, 155 

Taste compensations, 157 

Taste-bud, 110, 115 

Taste cells, 117 

Taste mixtures, 157 

Thirst, 179 



Sarasin, P., und F. Sarasin, 100, Thorns, H., and K. Nettesheim, 143, 

101 165 

Schaeffer, J. P., 24, 25, 41 Touch, 176 

Schiff, M., 43, 90, 134, 165 Tourtual, C. T., 57, 90 

Schirmer, R., 155, 165 Trigeminal nerve, 42 

Schmidt, 151 True odors > 44 
Schultze, M., 28, 29, 30, 31, 32, 41, Tuckerman, F., 110, 111, 115, 131 

58, 90, 175, 185 Von Uexkiill, J., 64, 90 

Schulze, F. E., 115, 130 Urbantschitsch, V., 165 
Schwalbe, G., 110, 114, 117, 118, 130 Valentin, G., 43, 53, 81, 85, 90, 91, 

Sense organs, 13, 18, 21 134, 165 

Seydel, O., 100, 101 Van Dam, C., 51, 91 

Sheldon, R. E., 64, 66, 90, 103, 104, Van der Stricht, O., 31, 41 



105, 109 

Sherrington, C. .S., 172, 185 
Shore, L. E., 149, 153, 165 
Sinuses, 25 
Smell and taste, 173 
Sour taste, 136, 150 
Specific energy of nerves, 152, 178 
Sphenoidal sinus, 26 
Sponges, 21 
Stabler, E. M., 55, 106, 141, 143, 165, 

171 

Stahr, H., 110, 111, 131 
Steiner, J., 64, 90 



Van Gehuchten, A., 35, 41 
Van Heusen, A. P., 148, 165 
Vaschide, N., 58, 60, 91, 162 
Vastarini-Cresi, G., 125, 131 
Vomero-nasal organs, 92, 181 
Veress, E., 58, 59, 60, 81, 91 
Von Vintschgau, M., 87, 131, 134, 

162, 165 
Von Vintschgau, M., und J. Honig- 

schmied, 124, 131, 155, 165 
Volta, 146 

Washburn, A. L., 179, 184 
Weber, E. H., 57, 58, 59, 91 



192 INDEX 

Winslow, C.-E. A., and D. Green- Zenneck, 134, 166 

berg, 69, 91 Von Zevne k, R., 147, 166 

Von Wittich, W., 154, 165 _ 
Woodrow, H., and B. Karpman, 49, Zu <* erkandl > E - "1 

91 Zuntz, N., 156, 166 

Wundt, W., 134, 166 Zwaardcmaker, H., 45, 47, 48, 50, 

Von Wyss, H., 114, 131 51, 57, 58, 59, 62, 70, 72, 73, 76, 

Zander, R., 124, 131 77, 85, 87, 91, 162